feat: Skills execution system (Phase 2B)

- SkillTool: execute skills by name via forked sub-agent
- BundledSkills: 4 built-in skills (verify, debug, review, commit)
- SkillLoader: bundled skills auto-loaded + budget-aware prompt listing
- /skills command: detail view with /skills <name>
- Budget management: 8K char budget, 250 char per-entry cap

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/java/com/claudecode/command/impl/SkillsCommand.java

@@ -31,6 +31,24 @@ public class SkillsCommand implements SlashCommand {
         SkillLoader loader = new SkillLoader(projectDir);
         List<SkillLoader.Skill> skills = loader.loadAll();
 
+        // If args provided, show specific skill detail
+        if (args != null && !args.isBlank()) {
+            String query = args.strip().toLowerCase();
+            var match = skills.stream()
+                    .filter(s -> s.name().equalsIgnoreCase(query))
+                    .findFirst();
+            if (match.isEmpty()) {
+                match = skills.stream()
+                        .filter(s -> s.name().toLowerCase().contains(query))
+                        .findFirst();
+            }
+            if (match.isPresent()) {
+                return formatSkillDetail(match.get());
+            }
+            return AnsiStyle.red("  Skill not found: " + args.strip()) + "\n"
+                    + AnsiStyle.dim("  Use /skills to list all available skills.");
+        }
+
         StringBuilder sb = new StringBuilder();
         sb.append("\n");
         sb.append(AnsiStyle.bold("  🎯 Available Skills\n"));
@@ -61,12 +79,43 @@ public class SkillsCommand implements SlashCommand {
                 if (!skill.whenToUse().isEmpty()) {
                     sb.append("    ").append(AnsiStyle.dim("When: " + skill.whenToUse())).append("\n");
                 }
-                sb.append("    ").append(AnsiStyle.dim("File: " + skill.filePath())).append("\n");
                 sb.append("\n");
             }
-            sb.append(AnsiStyle.dim("  Total " + skills.size() + " skills\n"));
+            sb.append(AnsiStyle.dim("  Total " + skills.size() + " skills"));
+            sb.append(AnsiStyle.dim(" • Use /skills <name> for details\n"));
         }
 
         return sb.toString();
     }
+
+    private String formatSkillDetail(SkillLoader.Skill skill) {
+        StringBuilder sb = new StringBuilder();
+        sb.append("\n");
+        sb.append(AnsiStyle.bold("  🎯 Skill: " + skill.name())).append("\n");
+        sb.append("  ").append("─".repeat(50)).append("\n\n");
+
+        sb.append("  ").append(AnsiStyle.bold("Source: ")).append(skill.source()).append("\n");
+        if (!skill.description().isEmpty()) {
+            sb.append("  ").append(AnsiStyle.bold("Description: ")).append(skill.description()).append("\n");
+        }
+        if (!skill.whenToUse().isEmpty()) {
+            sb.append("  ").append(AnsiStyle.bold("When to use: ")).append(skill.whenToUse()).append("\n");
+        }
+        sb.append("  ").append(AnsiStyle.bold("File: ")).append(skill.filePath()).append("\n");
+        sb.append("\n");
+
+        // Show content preview
+        String content = skill.content();
+        if (content.length() > 500) {
+            content = content.substring(0, 497) + "...";
+        }
+        sb.append(AnsiStyle.dim("  Content:\n"));
+        for (String line : content.lines().toList()) {
+            sb.append(AnsiStyle.dim("  │ " + line)).append("\n");
+        }
+        sb.append("\n");
+        sb.append(AnsiStyle.dim("  Tip: Ask AI to execute this skill or type: /verify, /debug, etc.\n"));
+
+        return sb.toString();
+    }
 }

src/main/java/com/claudecode/config/AppConfig.java

@@ -113,7 +113,8 @@ public class AppConfig {
                 new SleepTool(),
                 new ToolSearchTool(),
                 new EnterPlanModeTool(),
-                new ExitPlanModeTool()
+                new ExitPlanModeTool(),
+                new SkillTool()
         );
 
         // P2: 注册 MCP 工具桥接（将远程 MCP 工具映射为本地工具）
@@ -238,7 +239,7 @@ public class AppConfig {
     }
 
     @Bean
-    public String systemPrompt() {
+    public String systemPrompt(ToolContext toolContext) {
         Path projectDir = Path.of(System.getProperty("user.dir"));
 
         ClaudeMdLoader claudeLoader = new ClaudeMdLoader(projectDir);
@@ -248,6 +249,9 @@ public class AppConfig {
         skillLoader.loadAll();
         String skillsSummary = skillLoader.buildSkillsSummary();
 
+        // Inject SkillLoader into ToolContext for SkillTool
+        toolContext.set(SkillTool.SKILL_LOADER_KEY, skillLoader);
+
         GitContext gitContext = new GitContext(projectDir).collect();
         String gitSummary = gitContext.buildSummary();
 

src/main/java/com/claudecode/context/BundledSkills.java

@@ -0,0 +1,200 @@
+package com.claudecode.context;
+
+import com.claudecode.context.SkillLoader.Skill;
+
+import java.nio.file.Path;
+import java.util.List;
+
+/**
+ * 内置 Skills 注册 —— 对应 claude-code/src/skills/bundled/ 目录。
+ * <p>
+ * 提供默认可用的 Skills，无需用户手动创建 .md 文件。
+ */
+public final class BundledSkills {
+
+    private BundledSkills() {}
+
+    /**
+     * 获取所有内置 Skills。
+     */
+    public static List<Skill> getAll() {
+        return List.of(
+                verifySkill(),
+                debugSkill(),
+                reviewSkill(),
+                commitSkill()
+        );
+    }
+
+    /**
+     * verify — 代码验证 Skill。
+     * 运行项目测试、lint、类型检查，确保代码变更正确。
+     */
+    static Skill verifySkill() {
+        return new Skill(
+                "verify",
+                "Run tests and checks to verify code changes are correct",
+                "After making code changes that need verification",
+                """
+                # Verify Changes
+                
+                You are verifying that recent code changes are correct and don't break anything.
+                
+                ## Steps
+                
+                1. **Identify the project type** by reading build/config files:
+                   - Look for: package.json, pom.xml, build.gradle, Cargo.toml, go.mod, etc.
+                
+                2. **Run the appropriate test command**:
+                   - Node.js: `npm test` or `npx jest` or `npx vitest`
+                   - Java/Maven: `mvn test -q`
+                   - Java/Gradle: `./gradlew test`
+                   - Python: `pytest` or `python -m pytest`
+                   - Rust: `cargo test`
+                   - Go: `go test ./...`
+                
+                3. **Run linting if available**:
+                   - Node.js: `npm run lint` or `npx eslint .`
+                   - Python: `ruff check .` or `flake8`
+                   - Java: `mvn checkstyle:check -q`
+                
+                4. **Run type checking if available**:
+                   - TypeScript: `npx tsc --noEmit`
+                   - Python: `mypy .`
+                
+                5. **Report results**:
+                   - ✅ All checks passed
+                   - ❌ Failures found (include specific errors)
+                   - ⚠️ Warnings (include details)
+                
+                If a check fails, analyze the error and suggest a fix.
+                """,
+                "bundled",
+                Path.of("bundled://verify")
+        );
+    }
+
+    /**
+     * debug — 调试辅助 Skill。
+     * 分析错误信息，定位问题根因。
+     */
+    static Skill debugSkill() {
+        return new Skill(
+                "debug",
+                "Analyze errors and help debug issues",
+                "When encountering an error or unexpected behavior",
+                """
+                # Debug Issue
+                
+                You are helping debug an issue. Follow this systematic approach:
+                
+                ## Steps
+                
+                1. **Understand the error**:
+                   - Read the error message carefully
+                   - Identify the error type (compile, runtime, logic, etc.)
+                   - Note the file and line number if available
+                
+                2. **Reproduce the issue**:
+                   - Try to reproduce the error with the minimal command
+                   - Check if the error is consistent
+                
+                3. **Trace the root cause**:
+                   - Read the file(s) mentioned in the error
+                   - Check imports, dependencies, and configuration
+                   - Look for recent changes that might have caused it
+                   - Use Grep to find related code patterns
+                
+                4. **Analyze and diagnose**:
+                   - Identify the root cause (not just symptoms)
+                   - Consider edge cases and dependencies
+                   - Check if similar patterns exist elsewhere
+                
+                5. **Suggest fix**:
+                   - Propose a specific, minimal fix
+                   - Explain why the fix works
+                   - Note any side effects or risks
+                
+                If the user provides an error message, start from Step 1.
+                If the user describes unexpected behavior, start from Step 2.
+                """,
+                "bundled",
+                Path.of("bundled://debug")
+        );
+    }
+
+    /**
+     * review — 代码审查 Skill。
+     */
+    static Skill reviewSkill() {
+        return new Skill(
+                "review",
+                "Review code changes for quality, bugs, and best practices",
+                "When the user wants feedback on code changes",
+                """
+                # Code Review
+                
+                You are reviewing code changes. Focus on:
+                
+                ## Checklist
+                
+                1. **Correctness**: Does the code do what it's supposed to?
+                2. **Edge cases**: Are boundary conditions handled?
+                3. **Error handling**: Are errors caught and handled gracefully?
+                4. **Security**: Any injection, auth, or data exposure risks?
+                5. **Performance**: Any obvious performance issues?
+                6. **Readability**: Is the code clear and well-structured?
+                7. **Tests**: Are there tests? Do they cover key cases?
+                
+                ## Process
+                
+                1. Run `git diff` to see what changed
+                2. Read each changed file
+                3. For each issue found, note:
+                   - Severity: 🔴 Critical / 🟡 Warning / 🔵 Suggestion
+                   - File and line number
+                   - What's wrong and how to fix it
+                4. Summarize findings
+                """,
+                "bundled",
+                Path.of("bundled://review")
+        );
+    }
+
+    /**
+     * commit — 提交 Skill。
+     */
+    static Skill commitSkill() {
+        return new Skill(
+                "commit",
+                "Create a well-structured git commit with conventional commit message",
+                "When the user wants to commit changes",
+                """
+                # Git Commit
+                
+                Create a well-structured git commit.
+                
+                ## Steps
+                
+                1. Run `git status` and `git diff --stat` to see changes
+                2. Analyze the changes to determine:
+                   - Type: feat, fix, refactor, docs, test, chore, etc.
+                   - Scope: which module/area is affected
+                   - Breaking changes
+                3. Generate commit message in Conventional Commits format:
+                   ```
+                   type(scope): brief description
+                   
+                   Detailed body explaining what and why (not how).
+                   
+                   Breaking changes (if any).
+                   ```
+                4. Stage relevant files with `git add`
+                5. Create the commit with `git commit -m "..."`
+                6. Report the commit hash
+                """,
+                "bundled",
+                Path.of("bundled://commit")
+        );
+    }
+}

src/main/java/com/claudecode/context/SkillLoader.java

@@ -46,6 +46,10 @@ public class SkillLoader {
     public List<Skill> loadAll() {
         skills.clear();
 
+        // 0. 内置技能
+        skills.addAll(BundledSkills.getAll());
+        log.debug("Loaded {} bundled skills", BundledSkills.getAll().size());
+
         // 1. 用户级技能
         Path userSkillsDir = Path.of(System.getProperty("user.home"), ".claude", "skills");
         loadFromDirectory(userSkillsDir, "user");
@@ -147,25 +151,58 @@ public class SkillLoader {
     }
 
     /**
-     * 构建技能上下文摘要（注入系统提示词）
+     * 构建技能上下文摘要（注入系统提示词）。
+     * 支持预算控制，确保不超过上下文窗口的 1%。
      */
     public String buildSkillsSummary() {
+        return buildSkillsSummary(8000); // Default 8K chars budget
+    }
+
+    /**
+     * 构建技能上下文摘要（带预算控制）。
+     * 对应 TS formatCommandsWithinBudget()。
+     *
+     * @param charBudget 最大字符预算
+     */
+    public String buildSkillsSummary(int charBudget) {
         if (skills.isEmpty()) {
             return "";
         }
 
         StringBuilder sb = new StringBuilder();
         sb.append("# Available Skills\n\n");
+        sb.append("Skills can be invoked by name using the Skill tool or by typing /<name>.\n\n");
+
+        int budgetUsed = sb.length();
+        int perEntryMax = 250; // Per-entry cap for cache efficiency
+
         for (Skill skill : skills) {
-            sb.append("- **").append(skill.name()).append("**");
+            StringBuilder entry = new StringBuilder();
+            entry.append("- **").append(skill.name()).append("**");
             if (!skill.description().isEmpty()) {
-                sb.append(": ").append(skill.description());
+                String desc = skill.description();
+                if (desc.length() > perEntryMax - skill.name().length() - 10) {
+                    desc = desc.substring(0, perEntryMax - skill.name().length() - 13) + "...";
+                }
+                entry.append(": ").append(desc);
             }
             if (!skill.whenToUse().isEmpty()) {
-                sb.append(" (use when: ").append(skill.whenToUse()).append(")");
+                entry.append(" (use when: ").append(skill.whenToUse()).append(")");
+            }
+            entry.append("\n");
+
+            // Check budget
+            if (budgetUsed + entry.length() > charBudget) {
+                // Add truncation notice
+                sb.append("- ... and ").append(skills.size() - skills.indexOf(skill))
+                        .append(" more skills (use /skills to see all)\n");
+                break;
             }
-            sb.append("\n");
+
+            sb.append(entry);
+            budgetUsed += entry.length();
         }
+
         return sb.toString();
     }
 

src/main/java/com/claudecode/tool/impl/SkillTool.java

@@ -0,0 +1,198 @@
+package com.claudecode.tool.impl;
+
+import com.claudecode.context.SkillLoader;
+import com.claudecode.context.SkillLoader.Skill;
+import com.claudecode.tool.Tool;
+import com.claudecode.tool.ToolContext;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+
+/**
+ * Skill 执行工具 —— 对应 claude-code/src/tools/SkillTool/SkillTool.ts。
+ * <p>
+ * 通过名称调用已加载的 Skill。Skill 会作为 forked sub-agent 执行，
+ * 注入 Skill 的 markdown 内容作为上下文指导。
+ * <p>
+ * Skills 来源：
+ * <ul>
+ *   <li>用户级: ~/.claude/skills/</li>
+ *   <li>项目级: ./.claude/skills/</li>
+ *   <li>命令目录: ./.claude/commands/</li>
+ *   <li>内置 Skills: verify, debug 等</li>
+ * </ul>
+ */
+public class SkillTool implements Tool {
+
+    private static final Logger log = LoggerFactory.getLogger(SkillTool.class);
+
+    /** ToolContext key for SkillLoader */
+    public static final String SKILL_LOADER_KEY = "SKILL_LOADER";
+
+    @Override
+    public String name() {
+        return "Skill";
+    }
+
+    @Override
+    public String description() {
+        return """
+                Execute a registered skill by name. Skills are reusable, structured workflows \
+                defined in markdown files that guide a specialized sub-agent.
+                
+                When to use:
+                - User invokes a skill by name (e.g., /verify, /debug)
+                - You identify a task that matches a registered skill's "whenToUse" criteria
+                - Complex workflows that benefit from structured guidance
+                
+                Available skills are listed in the system prompt under "Available Skills".
+                
+                The skill runs as an isolated sub-agent with its own context. Provide any relevant \
+                arguments or context from the current conversation.
+                """;
+    }
+
+    @Override
+    public String inputSchema() {
+        return """
+                {
+                  "type": "object",
+                  "properties": {
+                    "skill_name": {
+                      "type": "string",
+                      "description": "Name of the skill to execute (case-insensitive)"
+                    },
+                    "arguments": {
+                      "type": "string",
+                      "description": "Arguments or context to pass to the skill"
+                    }
+                  },
+                  "required": ["skill_name"]
+                }
+                """;
+    }
+
+    @Override
+    public String execute(Map<String, Object> input, ToolContext context) {
+        String skillName = (String) input.get("skill_name");
+        String arguments = (String) input.getOrDefault("arguments", "");
+
+        if (skillName == null || skillName.isBlank()) {
+            return "Error: 'skill_name' is required";
+        }
+
+        // Get SkillLoader from context
+        SkillLoader skillLoader = context.get(SKILL_LOADER_KEY);
+        if (skillLoader == null) {
+            return "Error: SkillLoader not configured. No skills available.";
+        }
+
+        // Find skill by name
+        Optional<Skill> skillOpt = skillLoader.findByName(skillName);
+        if (skillOpt.isEmpty()) {
+            // Try partial match
+            skillOpt = findByPartialName(skillLoader.getSkills(), skillName);
+        }
+
+        if (skillOpt.isEmpty()) {
+            StringBuilder msg = new StringBuilder();
+            msg.append("Skill '").append(skillName).append("' not found.\n\n");
+            msg.append("Available skills:\n");
+            for (Skill s : skillLoader.getSkills()) {
+                msg.append("  - ").append(s.name());
+                if (!s.description().isEmpty()) {
+                    msg.append(": ").append(s.description());
+                }
+                msg.append("\n");
+            }
+            return msg.toString();
+        }
+
+        Skill skill = skillOpt.get();
+        log.info("Executing skill: {} [{}]", skill.name(), skill.source());
+
+        // Build skill execution prompt
+        String skillPrompt = buildSkillPrompt(skill, arguments);
+
+        // Execute via agent factory (same as AgentTool)
+        @SuppressWarnings("unchecked")
+        java.util.function.Function<String, String> agentFactory =
+                context.getOrDefault(AgentTool.AGENT_FACTORY_KEY, null);
+
+        if (agentFactory == null) {
+            // Fallback: return skill content for manual execution guidance
+            return "⚠️ Sub-agent not available. Skill content for manual execution:\n\n"
+                    + "# Skill: " + skill.name() + "\n"
+                    + skill.content();
+        }
+
+        try {
+            String result = agentFactory.apply(skillPrompt);
+            log.info("Skill '{}' completed, result: {} chars", skill.name(), result.length());
+            return result;
+        } catch (Exception e) {
+            log.debug("Skill execution failed", e);
+            return "Error executing skill '" + skill.name() + "': " + e.getMessage();
+        }
+    }
+
+    @Override
+    public boolean isReadOnly() {
+        return false; // Skills may modify files
+    }
+
+    @Override
+    public String activityDescription(Map<String, Object> input) {
+        String name = input != null ? (String) input.get("skill_name") : null;
+        return name != null ? "Running skill: " + name + "..." : "Running skill...";
+    }
+
+    /**
+     * Build the full prompt for skill execution.
+     */
+    private String buildSkillPrompt(Skill skill, String arguments) {
+        StringBuilder sb = new StringBuilder();
+        sb.append("You are executing a skill: ").append(skill.name()).append("\n\n");
+
+        if (!skill.description().isEmpty()) {
+            sb.append("Description: ").append(skill.description()).append("\n");
+        }
+        if (!skill.whenToUse().isEmpty()) {
+            sb.append("When to use: ").append(skill.whenToUse()).append("\n");
+        }
+        sb.append("\n");
+
+        // Inject skill content as instructions
+        sb.append("## Skill Instructions\n\n");
+        sb.append(skill.content()).append("\n\n");
+
+        // Inject arguments
+        if (arguments != null && !arguments.isBlank()) {
+            sb.append("## User Arguments\n\n");
+            sb.append(arguments).append("\n\n");
+        }
+
+        sb.append("""
+                ## Execution Guidelines
+                - Follow the skill instructions above carefully
+                - Use the available tools to complete the task
+                - Report results concisely when done
+                - If the skill requires user input, use AskUserQuestion
+                """);
+
+        return sb.toString();
+    }
+
+    /**
+     * Partial name match for skills.
+     */
+    private Optional<Skill> findByPartialName(List<Skill> skills, String name) {
+        String lower = name.toLowerCase();
+        return skills.stream()
+                .filter(s -> s.name().toLowerCase().contains(lower))
+                .findFirst();
+    }
+}