docs: document pi_embedded canary path and update state

docs/architecture/AGENT_DIAGRAM.md

@@ -31,6 +31,7 @@ flowchart LR
     SM[SessionManager\nSQLite]
     OR[AgentOrchestrator]
     NA[NativeAgent\n(tool loop)]
+    EB[Optional External Backends\nclaude_code/opencode/codex/gemini/pi_embedded]
     MR[ModelRouter]
     TP[ToolPolicy + ToolRegistry]
     TE[ToolExecutor\nhooks + enforcement + audit]
@@ -60,7 +61,9 @@ flowchart LR
   CA --> RT
   RT --> SM
   RT --> OR
+  RT --> EB
   OR --> NA
+  EB --> MP
   NA --> MR
   MR --> MP
 
@@ -107,6 +110,9 @@ ChannelAdapter  ->  ChannelRegistry
         |               v
         |          ModelClient
         |
+        +----> (optional, non-tool turns) ExternalBackend
+                         (claude_code/opencode/codex/gemini/pi_embedded)
+        |
         +----> (optional) PairingManager gate for unknown senders
 
 Tool Calls (inside NativeAgent loop)
@@ -130,6 +136,7 @@ Key files:
 - Routing + per-session agent creation: `src/daemon/routing.ts`
 - Orchestration: `src/backends/native/orchestrator.ts`
 - Tool loop: `src/backends/native/agent.ts`
+- 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`
 

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