feat: add persisted manual pi backend mode controls

docs/architecture/AGENT_DIAGRAM.md

@@ -28,6 +28,7 @@ flowchart LR
     CA[ChannelAdapters]
     GW[Gateway\nHTTP + WS JSON-RPC + Web UI]
     RT[Routing\ncreateMessageRouter()]
+    PF[Preferences\n~/.local/share/flynn/preferences.json\nmodelTier + backendMode]
     SM[SessionManager\nSQLite]
     OR[AgentOrchestrator]
     NA[NativeAgent\n(tool loop)]
@@ -55,6 +56,7 @@ flowchart LR
 
   CG --> RT
   CE --> RT
+  PF --> RT
 
   CH --> CA
   GW --> RT
@@ -96,6 +98,9 @@ ChannelAdapter  ->  ChannelRegistry
         |               v
         |        createMessageRouter()
         |               |
+        |               +----> Runtime backend mode overrides
+        |                      (/backend status|activate pi|deactivate pi|use config)
+        |               |
         |               v
         |         SessionManager
         |               |
@@ -134,6 +139,7 @@ Outbound Reply
 Key files:
 
 - Routing + per-session agent creation: `src/daemon/routing.ts`
+- Runtime preference persistence (`modelTier`, `backendMode`): `src/preferences.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`