Resolves #13464

This hardens the Knowledge Base MCP health/gate path for unified Chroma. A stale resolved KB collection handle now gets invalidated and retried once when count() returns a Chroma not-found/resource-not-found error; connected Chroma with unavailable corpus now reports degraded instead of a daemon-level unhealthy; provider readiness is still evaluated; and manage_knowledge_base is exempted from the full corpus-health gate so recovery/sync can run.

Evidence: L2 (focused unit tests for stale-handle retry, degraded health payload, and recovery-tool exemption) -> L2 required (close-target ACs are service/MCP contract behavior covered by unit tests). No residuals.

Deltas from ticket

• Kept the payload shape compatible and documented the revised health semantics in openapi.yaml.
• Did not implement a cross-process invalidation bus or restore/rebuild the KB corpus; those remain explicitly out of scope.

Test Evidence

• npm run test-unit -- test/playwright/unit/ai/services/knowledge-base/HealthService.providerReady.spec.mjs test/playwright/unit/ai/services/knowledge-base/ChromaManager.spec.mjs test/playwright/unit/ai/mcp/server/knowledge-base/Server.spec.mjs -> 20 passed.
• git diff --check -> passed.
• Commit hook passed whitespace, shorthand, AiConfig mutation, JSDoc types, and ticket archaeology checks.

Post-Merge Validation

• Restart/reconnect the KB MCP runtime and rerun healthcheck to confirm live diagnostics use the updated degraded/healthy semantics against the current corpus state.
• Track KB corpus restoration/root-cause separately, per #13464 out-of-scope.

Commits

• 90171b499 — fix(ai): recover KB stale collection handles (#13464)

Authored by Euclid (GPT-5 Codex, Codex Desktop). Session 4ce60429-2986-4543-be2d-741957c75b6c.