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/storage/thoughts.go

@@ -338,14 +338,19 @@ func (s *Store) UpdateThoughtTriage(userID, thoughtID, action, convertTo, target
 				})
 			}
 			if len(thought.Subtasks) > 0 {
-				for _, st := range thought.Subtasks {
+				for i, st := range thought.Subtasks {
+					id := domain.NewID("blk")
 					blocks = append(blocks, domain.DocumentBlock{
-						ID:      domain.NewID("blk"),
+						ID:      id,
 						Type:    "todo",
 						Content: st.Title,
 						Checked: st.IsCompleted,
 						Level:   1,
 					})
+					// A task stays linked: its subtasks are tied to these checkboxes (task_doc_sync.go)
+					if isTask {
+						thought.Subtasks[i].BlockID = id
+					}
 				}
 			}
 			var metaItems []string
@@ -380,6 +385,8 @@ func (s *Store) UpdateThoughtTriage(userID, thoughtID, action, convertTo, target
 					newStatus = domain.StatusKept
 					convertedType = "document_block"
 					convertedRefID = doc.ID + ":" + blocks[0].ID
+					subsJSON, _ := json.Marshal(thought.Subtasks)
+					_, _ = s.db.Exec(`UPDATE thoughts SET subtasks = ? WHERE id = ?`, string(subsJSON), thought.ID)
 				}
 			}
 		} else if convertTo == "list_item" && targetListID != "" {
@@ -475,7 +482,7 @@ func (s *Store) UpdateThoughtDetails(userID, thoughtID string, content *string,
 		thought.Notes = *notes
 	}
 	if subtasks != nil {
-		thought.Subtasks = *subtasks
+		thought.Subtasks = keepSubtaskLinks(thought.Subtasks, *subtasks)
 	}
 
 	tagsJSON, _ := json.Marshal(thought.Tags)