docs(plan): add pi canary evaluation gate and tracking (diagram review no topology change)

docs/architecture/AGENT_DIAGRAM.md

@@ -139,6 +139,7 @@ Key files:
 - External backend adapters: `src/backends/external.ts`, `src/backends/piEmbedded.ts`
 - Model routing: `src/models/router.ts`
 - Tool policy + execution: `src/tools/policy.ts`, `src/tools/executor.ts`
+- Canary backend telemetry summarization (offline evaluation): `src/audit/backendCanarySummary.ts`, `scripts/summarize-backend-canary.ts`
 
 ## Component Graph (Agent-Safety Boundary)
 

docs/architecture/GATEWAY_SESSIONS_AND_QUEUE.md

@@ -11,6 +11,7 @@ If you only want the protocol surface, see `docs/api/PROTOCOL.md`.
 - Agent work is queued per `sessionId` (FIFO), not per connection.
 - Sessions persist in SQLite via `SessionManager` even if clients disconnect.
 - Once dequeued, message routing may execute the native orchestrator path or an optional external backend path (`claude_code`, `opencode`, `codex`, `gemini`, `pi_embedded`) depending on agent/backend config.
+- Backend routing outcomes are auditable via `backend.route` / `backend.success` / `backend.fallback`, which enables offline canary evaluation without changing gateway protocol methods.
 
 ## Component Map