feat(worker): dependsOn via MCP + honest staleness signal in merge preview

Adds a user/MCP-declared task dependency (DependsOnTaskId) distinct from the
planning chain's internal BlockedByTaskId: add_task/update_task can set it,
the queue picker skips a Queued task until the dependency reaches Done, a
Failed/Cancelled dependency leaves the dependent blocked instead of starving
silently, and setting a link rejects self-reference/unknown-id/cycles.
get_task/list_tasks/batch_get_tasks now report blocked/blockedReason, and
wait_for_task_change reports "Blocked" immediately instead of running out its
timeout on a task the picker will never claim.

preview_merge/preview_merge_set gain staleFiles: files a branch touches that
the target branch also changed since the branch's fork point, a more honest
staleness signal than `behind` alone.

src/ClaudeDo.Worker/External/BatchMcpTools.cs

@@ -102,7 +102,7 @@ public sealed class BatchMcpTools
             {
                 var created = await _svc.AddTask(
                     listId, item.Title, item.Description, createdBy,
-                    queueImmediately, item.Model, cancellationToken);
+                    queueImmediately, item.Model, cancellationToken: cancellationToken);
                 results.Add(new BatchAddTaskResult(i, item.Title, true, created.Task, created.PossibleDuplicates, null));
             }
             catch (OperationCanceledException) { throw; }