Phase 003: remove the coding-agent and ekko-agent runtimes

Every chat session now runs through the Hermes bridge, so the three chat
chains become feature-equivalent instead of three partially-maintained
dispatch paths. Keeping the coding-agent and in-house agent runtimes alive
meant every bridge change had to be mirrored into two more runtimes that
had already drifted.

Historical rows are read, not migrated: sessions stored with the retired
source normalize to cli on load, workflow graphs naming the retired agents
keep loading and run on the bridge, and the visible-source check plus
SOURCE_LABELS still recognize the old value so those sessions stay listed.

The public surface follows the code: the OpenAPI generator and the MCP
chat-run tool schema drop the six retired entry fields so external callers
are not told about runtimes that no longer exist.

harness-check gains a RETIRED_RUNTIME scan so the residual check is a
mechanical gate instead of a manual grep, with the two surviving read-side
compat sites recorded with reasons.

Refs: docs/phases/phase_003.md

tests/server/api-docs-controller.test.ts

@@ -30,6 +30,6 @@ describe('api docs controller', () => {
     )
     expect(
       ctx.body.paths['/api/chat-run/runs'].post.requestBody.content['application/json'].schema.properties.source.enum,
-    ).toEqual(['cli', 'coding_agent', 'global_agent'])
+    ).toEqual(['cli', 'global_agent', 'workflow'])
   })
 })