chat: rename the pi chat transport + model list to chat (Stage 4b/1)

Pure rename, no behavior change. Moves the misnamed "pi" chat harness into the
chat namespace:

- api/pi/{websocket,session-manager,types,logger,list-models} → api/chat/
- merge api/pi/rest.ts into api/chat/chat.ts (/pi/models → /chat/models,
  /pi/stt → /chat/stt); drop the piRestRouter mount
- PiEvent → ChatEvent, piWebsocket → chatWebsocket, listPiModels → listChatModels
- WS route /api/pi/chat/ws → /api/chat/ws, provider tag 'pi' → 'chat'
- frontend: useChat/useAudioRecording URLs, usePiModels→useModels /
  useVisiblePiModels→useVisibleModels / useEnabledPiModels→useEnabledModels,
  'PI_MODELS' query key → 'CHAT_MODELS', attachments provider 'pi-mono' → 'chat'

The /pi-mono provider/harness settings router is renamed separately (next commit).
Note: the WS route change requires the mobile app to point at /api/chat/ws.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/sidecar/protocol.ts

@@ -1,4 +1,4 @@
-import type { MessageCost, PiEvent } from '../api/pi/types';
+import type { MessageCost, ChatEvent } from '../api/chat/types';
 
 // ── Envelope ──
 
@@ -29,7 +29,7 @@ export type SidecarEvent =
   | { type: 'proxy:secret'; id: string; secret: string }
   // Claude Code
   | { type: 'claude:spawned'; id: string; sessionKey: string }
-  | { type: 'claude:event'; sessionKey: string; event: PiEvent }
+  | { type: 'claude:event'; sessionKey: string; event: ChatEvent }
   | { type: 'claude:result'; id: string; result: ClaudeCodeResult }
   | { type: 'claude:error'; id: string; error: string }
   | { type: 'claude:killed'; id: string }