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/tasks/pipeline-executor.ts

@@ -6,10 +6,10 @@ import { tmpdir } from 'node:os';
 import { getUserSettings } from 'officerdb';
 import { getTaskByDirName } from './task-files';
 import { getHomeDirForRole, getHomeDir } from '../../data-path';
-import { resolveBaseCwd } from '../pi/websocket';
+import { resolveBaseCwd } from '../chat/websocket';
 import { SANDBOX_HOME } from '../../sidecar/sandbox';
 import { sendClaudeCodeStreaming } from '../../channels/send-claude-code';
-import type { PiEvent, MessageCost } from '../pi/types';
+import type { ChatEvent, MessageCost } from '../chat/types';
 import * as jobManager from './pipeline-job-manager';
 
 const DEFAULT_MODEL = 'claude-code';
@@ -117,7 +117,7 @@ async function runAgenticStep({ userId, email, username, role, taskDirName, prom
       fn();
     };
 
-    const onEvent = (event: PiEvent) => {
+    const onEvent = (event: ChatEvent) => {
       if (abortSignal.aborted) return;
       lastActivity = Date.now();