refactor: split FileBrowserPane / Workspace / runAssistantTurn

- FileBrowserPane.tsx: deleted (unreferenced post-v1.4 PaneTab.tsx removal;
  the legacy file_browser pane kind isn't part of the active WorkspacePane
  taxonomy).
- Workspace.tsx (524 -> 172 lines): extracted useWorkspacePanes(sessionId)
  and useSessionChats(sessionId) hooks. Workspace is layout-only composition
  now. localStorage key + WS frame handling + drag semantics unchanged.
- inference.ts runAssistantTurn (~265 -> 48 lines): bundled args into
  TurnArgs interface, extracted executeStreamPhase / executeToolPhase /
  finalizeCompletion / handleAbortOrError. All WS publish ordering preserved
  byte-for-byte (mentally traced for tool / non-tool / abort / error /
  depth-exceeded paths). flushPromise chain + setImmediate + signal
  propagation unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/useSessionChats.ts

@@ -0,0 +1,175 @@
+import { useCallback, useEffect, useRef, useState } from 'react';
+import { toast } from 'sonner';
+import { api } from '@/api/client';
+import type { Chat } from '@/api/types';
+import { sessionEvents } from '@/hooks/sessionEvents';
+
+export interface UseSessionChatsOpts {
+  removeChatFromPanes: (chatId: string) => void;
+  openChatInPane: (paneIdx: number, chatId: string) => void;
+  // Thin wrapper around openChatInPane(activePaneIdxRef.current, chatId);
+  // built by Workspace and passed in so this hook doesn't need to know
+  // about pane indexing.
+  openChatInActivePane: (chatId: string) => void;
+  initializeFirstChatIfEmpty: (chatId: string) => void;
+}
+
+export interface UseSessionChatsResult {
+  chats: Chat[];
+  setChats: React.Dispatch<React.SetStateAction<Chat[]>>;
+  createChat: (paneIdx: number) => Promise<void>;
+  archiveChat: (chatId: string) => Promise<void>;
+  unarchiveChat: (chatId: string) => Promise<void>;
+  deleteChat: (chatId: string) => Promise<void>;
+  renameChat: (chatId: string, name: string) => Promise<void>;
+  handleLandingSend: (paneIdx: number, content: string) => Promise<void>;
+}
+
+export function useSessionChats(
+  sessionId: string,
+  opts: UseSessionChatsOpts,
+): UseSessionChatsResult {
+  const [chats, setChats] = useState<Chat[]>([]);
+  const chatsRef = useRef<Chat[]>([]);
+  chatsRef.current = chats;
+
+  // Stable refs to opts callbacks so the subscription effect — which only
+  // re-runs on sessionId change — always sees the latest closures without
+  // unsubscribe/resubscribe churn.
+  const removeChatFromPanesRef = useRef(opts.removeChatFromPanes);
+  removeChatFromPanesRef.current = opts.removeChatFromPanes;
+  const openChatInPaneRef = useRef(opts.openChatInPane);
+  openChatInPaneRef.current = opts.openChatInPane;
+  const openChatInActivePaneRef = useRef(opts.openChatInActivePane);
+  openChatInActivePaneRef.current = opts.openChatInActivePane;
+  const initializeFirstChatIfEmptyRef = useRef(opts.initializeFirstChatIfEmpty);
+  initializeFirstChatIfEmptyRef.current = opts.initializeFirstChatIfEmpty;
+
+  useEffect(() => {
+    let cancelled = false;
+    api.chats.listForSession(sessionId).then((list) => {
+      if (cancelled) return;
+      setChats(list);
+      const openChat = list.find((c) => c.status === 'open');
+      if (openChat) {
+        initializeFirstChatIfEmptyRef.current(openChat.id);
+      }
+    }).catch(() => {});
+    return () => { cancelled = true; };
+  }, [sessionId]);
+
+  useEffect(() => {
+    return sessionEvents.subscribe((event) => {
+      if (event.type === 'chat_created' && event.session_id === sessionId) {
+        setChats((prev) => {
+          if (prev.some((c) => c.id === event.chat.id)) return prev;
+          return [event.chat, ...prev];
+        });
+      }
+      if (event.type === 'chat_updated') {
+        setChats((prev) => prev.map((c) =>
+          c.id === event.chat_id ? { ...c, name: event.name, updated_at: event.updated_at } : c
+        ));
+      }
+      if (event.type === 'chat_archived') {
+        setChats((prev) => prev.map((c) =>
+          c.id === event.chat_id ? { ...c, status: 'archived' as const } : c
+        ));
+        removeChatFromPanesRef.current(event.chat_id);
+      }
+      if (event.type === 'chat_unarchived') {
+        setChats((prev) => {
+          if (prev.some((c) => c.id === event.chat.id)) {
+            return prev.map((c) => c.id === event.chat.id ? { ...c, status: 'open' as const } : c);
+          }
+          return [event.chat, ...prev];
+        });
+      }
+      if (event.type === 'chat_deleted') {
+        setChats((prev) => prev.filter((c) => c.id !== event.chat_id));
+        removeChatFromPanesRef.current(event.chat_id);
+      }
+      if (event.type === 'open_chat_in_active_pane') {
+        openChatInActivePaneRef.current(event.chat_id);
+      }
+    });
+  }, [sessionId]);
+
+  const createChat = useCallback(async (paneIdx: number) => {
+    try {
+      const chat = await api.chats.create(sessionId);
+      // Optimistic local insert; the WS chat_created echo will be deduped by id.
+      setChats((prev) => {
+        if (prev.some((c) => c.id === chat.id)) return prev;
+        return [chat, ...prev];
+      });
+      openChatInPaneRef.current(paneIdx, chat.id);
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : 'Failed to create chat');
+    }
+  }, [sessionId]);
+
+  const archiveChat = useCallback(async (chatId: string) => {
+    try {
+      await api.chats.archive(chatId);
+      // Server publishes chat_archived; bus forwarder updates state.
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : 'Failed to archive chat');
+    }
+  }, []);
+
+  const unarchiveChat = useCallback(async (chatId: string) => {
+    try {
+      await api.chats.unarchive(chatId);
+      // Server publishes chat_unarchived.
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : 'Failed to restore chat');
+    }
+  }, []);
+
+  const deleteChat = useCallback(async (chatId: string) => {
+    try {
+      await api.chats.remove(chatId);
+      setChats((prev) => prev.filter((c) => c.id !== chatId));
+      removeChatFromPanesRef.current(chatId);
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : 'Failed to delete chat');
+    }
+  }, []);
+
+  const renameChat = useCallback(async (chatId: string, name: string) => {
+    try {
+      await api.chats.update(chatId, { name });
+      setChats((prev) => prev.map((c) =>
+        c.id === chatId ? { ...c, name } : c
+      ));
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : 'Failed to rename chat');
+    }
+  }, []);
+
+  const handleLandingSend = useCallback(async (paneIdx: number, content: string) => {
+    try {
+      const chat = await api.chats.create(sessionId);
+      setChats((prev) => {
+        if (prev.some((c) => c.id === chat.id)) return prev;
+        return [chat, ...prev];
+      });
+      openChatInPaneRef.current(paneIdx, chat.id);
+      await api.messages.send(chat.id, content);
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : 'Failed to send');
+    }
+  }, [sessionId]);
+
+  return {
+    chats,
+    setChats,
+    createChat,
+    archiveChat,
+    unarchiveChat,
+    deleteChat,
+    renameChat,
+    handleLandingSend,
+  };
+}