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/api/chat/list-models.ts

@@ -0,0 +1,16 @@
+import type { ModelInfo } from './types';
+
+// Claude-only. The runner is the `claude` CLI, so the model list is a fixed set of Claude tiers.
+const CLAUDE_CODE_MODELS: ModelInfo[] = [
+  { id: 'claude-code/opus', name: 'opus', provider: 'claude-code', contextWindow: 200000, maxTokens: 16000, reasoning: true, images: true },
+  { id: 'claude-code/sonnet', name: 'sonnet', provider: 'claude-code', contextWindow: 200000, maxTokens: 16000, reasoning: true, images: true },
+  { id: 'claude-code/haiku', name: 'haiku', provider: 'claude-code', contextWindow: 200000, maxTokens: 8192, reasoning: false, images: true },
+];
+
+export function invalidateModelCache(): void {
+  // No-op: the Claude model list is static. Kept for call-site compatibility.
+}
+
+export async function listChatModels(): Promise<ModelInfo[]> {
+  return [...CLAUDE_CODE_MODELS];
+}