feat(tasks): linked tasks sync title and subtasks with their note in both directions

Subtasks of a task linked to a note checkbox are tied 1:1 to checkboxes of that note
(subtasks[].blockId), wherever they are moved. ReconcileDocTaskLinks now carries tick, title and
subtasks from the note to the task and adopts new checkboxes directly under the task;
SyncTaskToDoc carries tick, title and subtasks to the note and adds checkboxes for new subtasks.
Deleting is not mirrored. Deepening a task ties its subtasks right away; saves without links keep
the server's links. The sync response reports taskLinks/subtaskLinks; the editor shows an
"Unteraufgabe" badge. v0.36.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/handlers_thoughts.go

@@ -320,7 +320,7 @@ func (s *Server) updateTask(userID, thoughtID string, req taskPatch) (*domain.Th
 		}
 		updated.Assignees = assignees
 	}
-	if docID := s.store.SyncDocBlockAndTask(userID, thoughtID, updated.IsCompleted); docID != "" {
+	if docID := s.store.SyncTaskToDoc(userID, thoughtID); docID != "" {
 		_, head, _ := s.store.GetDocOpsSince(docID, 0, 0)
 		s.notifyDocChanged(docID, head)
 	}