docs: document pi_embedded canary path and update state

docs/architecture/GATEWAY_SESSIONS_AND_QUEUE.md

@@ -10,6 +10,7 @@ If you only want the protocol surface, see `docs/api/PROTOCOL.md`.
 - Each connection is attached to a `sessionId`.
 - 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.
 
 ## Component Map
 
@@ -30,7 +31,7 @@ flowchart LR
   subgraph CORE[Flynn Core]
     SM[SessionManager\nin-memory cache + SQLite]
     SS[SessionStore\nSQLite tables]
-    AO[AgentOrchestrator]
+    AO[AgentOrchestrator / External Backends]
   end
 
   WS --> GS