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/session-manager.ts

@@ -0,0 +1,152 @@
+import type { UserSession } from "./types";
+import { logger } from "./logger";
+
+class SessionManager {
+  private sessions = new Map<string, UserSession>();
+  private userSessions = new Map<string, string[]>();
+
+  getOrCreate(
+    sessionId: string,
+    email: string,
+    cwd: string,
+    model: string,
+    groupSlug?: string | null,
+    context?: string,
+    contextId?: string,
+  ): UserSession {
+    let session = this.sessions.get(sessionId);
+
+    if (!session) {
+      session = {
+        sessionId,
+        email,
+        cwd,
+        model,
+        piProcess: null,
+        ws: null,
+        lastActivity: Date.now(),
+        idleTimer: null,
+        streamBuffer: "",
+        isGenerating: false,
+        systemContextSent: false,
+        messages: [],
+        meta: {
+          id: sessionId,
+          title: "",
+          model,
+          cwd,
+          groupSlug: groupSlug || null,
+          context,
+          contextId,
+          createdAt: Date.now(),
+          updatedAt: Date.now(),
+          messageCount: 0,
+          cost: {
+            inputTokens: 0,
+            outputTokens: 0,
+            totalUSD: 0,
+          },
+        },
+      };
+
+      this.sessions.set(sessionId, session);
+
+      const userSessionIds = this.userSessions.get(email) || [];
+      userSessionIds.push(sessionId);
+      this.userSessions.set(email, userSessionIds);
+    }
+
+    session.lastActivity = Date.now();
+    return session;
+  }
+
+  getSession(sessionId: string): UserSession | null {
+    return this.sessions.get(sessionId) || null;
+  }
+
+  getUserSessions(email: string): UserSession[] {
+    const sessionIds = this.userSessions.get(email) || [];
+    return sessionIds
+      .map((id) => this.sessions.get(id))
+      .filter((s): s is UserSession => s !== undefined);
+  }
+
+  deleteSession(sessionId: string): void {
+    const session = this.sessions.get(sessionId);
+    if (!session) return;
+
+    if (session.idleTimer) {
+      clearTimeout(session.idleTimer);
+    }
+
+    // Clean up sidecar subscriptions
+    if (session._sidecarUnsub) {
+      session._sidecarUnsub();
+    }
+    if (session._claudeKill) {
+      session._claudeKill();
+    }
+
+    this.sessions.delete(sessionId);
+
+    const userSessionIds = this.userSessions.get(session.email);
+    if (userSessionIds) {
+      const filtered = userSessionIds.filter(
+        (id) => id !== sessionId
+      );
+      if (filtered.length > 0) {
+        this.userSessions.set(session.email, filtered);
+      } else {
+        this.userSessions.delete(session.email);
+      }
+    }
+  }
+
+  attachWs(sessionId: string, ws: any): void {
+    const session = this.sessions.get(sessionId);
+    if (session) {
+      session.ws = ws;
+      session.lastActivity = Date.now();
+
+      if (session.idleTimer) {
+        clearTimeout(session.idleTimer);
+        session.idleTimer = null;
+      }
+    }
+  }
+
+  detachWs(sessionId: string): void {
+    const session = this.sessions.get(sessionId);
+    if (session) {
+      session.ws = null;
+      session.lastActivity = Date.now();
+    }
+  }
+
+  setIdleTimeout(sessionId: string, timeoutMs: number): void {
+    const session = this.sessions.get(sessionId);
+    if (!session) return;
+
+    if (session.idleTimer) {
+      clearTimeout(session.idleTimer);
+    }
+
+    session.idleTimer = setTimeout(() => {
+      logger.info('Session idle timeout reached, cleaning up', { sessionId, timeoutMs });
+      this.deleteSession(sessionId);
+    }, timeoutMs);
+  }
+
+  updateActivity(sessionId: string): void {
+    const session = this.sessions.get(sessionId);
+    if (session) {
+      session.lastActivity = Date.now();
+    }
+  }
+
+  getAllSessions(): UserSession[] {
+    return Array.from(this.sessions.values());
+  }
+}
+
+export const sessionManager = new SessionManager();