fix(offline-sync): keep page titles in sync between REST and Yjs

Title now lives in the page's Yjs 'title' fragment, but two paths corrupted it:

- Rename-revert: a REST/MCP title change wrote only the page.title column,
  never the Yjs fragment, so the next editor open replayed the stale Yjs title
  and reverted the rename. PageService.update now mirrors the new title into the
  Yjs 'title' fragment via CollaborationGateway.writePageTitle, which goes
  through openDirectConnection directly (Redis-independent: works with
  COLLAB_DISABLE_REDIS and in single-process deployments, unlike the
  Redis-routed handleYjsEvent path). The write is best-effort: a Yjs failure is
  logged and never rolls back the committed column write. Agent provenance
  (actor/aiChatId) is threaded into the store context.

- Untitled-on-open: an empty/just-initialized 'title' fragment clobbered a
  non-empty page.title to '' on open. onStoreDocument now treats the title as
  changed only when the extracted text is non-empty, covering both the
  title-only and body+title save branches. Empty-retitling via collab is
  intentionally impossible; the REST DTO is the place to enforce non-empty.

writeTitleFragment does a full clear+seed of the 'title' fragment (no
duplication/concatenation) and leaves the body fragment intact. Removed the dead
useTreeMutation.handleRename path. Adds unit tests for writeTitleFragment, the
gateway write, the anti-empty-clobber guard, and agent provenance.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/page/tree/hooks/use-tree-mutation.ts

@@ -14,7 +14,6 @@ import {
   useCreatePageMutation,
   useRemovePageMutation,
   useMovePageMutation,
-  useUpdatePageMutation,
   updateCacheOnMovePage,
 } from "@/features/page/queries/page-query.ts";
 import { buildPageUrl } from "@/features/page/page.utils.ts";
@@ -26,7 +25,6 @@ export type UseTreeMutation = {
     parentId: string | null,
     opts?: { temporary?: boolean },
   ) => Promise<void>;
-  handleRename: (id: string, name: string) => Promise<void>;
   handleDelete: (id: string) => Promise<void>;
 };
 
@@ -38,7 +36,6 @@ export function useTreeMutation(spaceId: string): UseTreeMutation {
   // children) and then immediately invokes a handler.
   const store = useStore();
   const createPageMutation = useCreatePageMutation();
-  const updatePageMutation = useUpdatePageMutation();
   const removePageMutation = useRemovePageMutation();
   const movePageMutation = useMovePageMutation();
   const navigate = useNavigate();
@@ -192,20 +189,6 @@ export function useTreeMutation(spaceId: string): UseTreeMutation {
     [spaceId, createPageMutation, setData, store, navigate, spaceSlug],
   );
 
-  const handleRename = useCallback(
-    async (id: string, name: string) => {
-      setData((prev) =>
-        treeModel.update(prev, id, { name } as Partial<SpaceTreeNode>),
-      );
-      try {
-        await updatePageMutation.mutateAsync({ pageId: id, title: name });
-      } catch (error) {
-        console.error("Error updating page title:", error);
-      }
-    },
-    [updatePageMutation, setData],
-  );
-
   const handleDelete = useCallback(
     async (id: string) => {
       const node = treeModel.find(
@@ -251,7 +234,7 @@ export function useTreeMutation(spaceId: string): UseTreeMutation {
     [removePageMutation, setData, store, pageSlug, navigate, spaceSlug],
   );
 
-  return { handleMove, handleCreate, handleRename, handleDelete };
+  return { handleMove, handleCreate, handleDelete };
 }
 
 function isPageInNode(node: SpaceTreeNode, pageSlug: string): boolean {