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/channels/discord/handler.ts

@@ -3,11 +3,11 @@ import { findUserByIntegrationConfig, readConfigValue } from 'officerdb';
 import { sendAndAwait, getSessionModel, setSessionModel } from '../send-and-await';
 import { consumePairingCode } from '../pairing';
 import { chunkMessage } from './chunker';
-import { listPiModels } from '@@/api/pi/list-models';
+import { listChatModels } from '@@/api/chat/list-models';
 import { enqueueJob } from '../../queue/init';
 import { readJob } from '@@/queue/storage';
 import { openUserEmailDb } from '@@/api/email/email-db';
-import type { ModelInfo } from '@@/api/pi/types';
+import type { ModelInfo } from '@@/api/chat/types';
 import { toShellUsername } from '@@/data-path';
 
 const PAIRING_CODE_PATTERN = /^[A-Z0-9]{6}$/;
@@ -19,7 +19,7 @@ type AccessPolicy = { allowedModels: string[] };
 const ACCESS_POLICY_KEY = 'pi-access-policy';
 
 async function getVisibleModels(): Promise<ModelInfo[]> {
-  const allModels = await listPiModels();
+  const allModels = await listChatModels();
   const policy = await readConfigValue<AccessPolicy>(ACCESS_POLICY_KEY, { allowedModels: [] });
   const allowed = policy.allowedModels;