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/client/api.test.ts

@@ -173,13 +173,13 @@ describe('API Client', () => {
         status: 500,
         text: () => Promise.resolve(JSON.stringify({
           error: {
-            message: 'spawn claude ENOENT',
+            message: 'bridge connect ENOENT',
             code: 'ENOENT',
           },
         })),
       })
 
-      await expect(request('/api/coding-agents/runs/session-1/input')).rejects.toThrow('API Error 500: spawn claude ENOENT')
+      await expect(request('/api/hermes/sessions/session-1/input')).rejects.toThrow('API Error 500: bridge connect ENOENT')
     })
 
     it('returns parsed JSON on success', async () => {