feat: add persisted manual pi backend mode controls

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.
+- Runtime backend mode can be overridden manually via `/backend` command fast-path (`status`, `activate pi`, `deactivate pi`, `use config`) and is persisted in preferences.
 - Backend routing outcomes are auditable via `backend.route` / `backend.success` / `backend.fallback`, which enables offline canary evaluation without changing gateway protocol methods.
 
 ## Component Map
@@ -19,6 +20,7 @@ If you only want the protocol surface, see `docs/api/PROTOCOL.md`.
 flowchart LR
   subgraph CFG[Config + Runtime Policy]
     QP[server.queue policy\nmode/cap/overflow/overrides]
+    BM[backend runtime mode\nconfig_default|force_native|force_pi_embedded]
   end
 
   subgraph GW[Gateway Process]
@@ -37,6 +39,7 @@ flowchart LR
 
   WS --> GS
   QP --> GS
+  BM --> GS
   GS --> LQ
   GS --> SB
   LQ --> AQ