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.Data/Models/TaskEntity.cs

@@ -28,6 +28,13 @@ public sealed class TaskEntity
     public TaskStatus Status { get; set; } = TaskStatus.Idle;
     public PlanningPhase PlanningPhase { get; set; } = PlanningPhase.None;
     public string? BlockedByTaskId { get; set; }
+
+    // A user/MCP-declared predecessor, distinct from BlockedByTaskId (the planning chain's own
+    // internal link): the picker also skips a Queued task while this is set and the referenced
+    // task's Status isn't Done. Unlike the chain, a Failed/Cancelled dependency does NOT cascade
+    // or auto-resolve -- the dependent just stays blocked and reports why (see TaskStateService.
+    // SetDependsOnAsync and QueuePicker).
+    public string? DependsOnTaskId { get; set; }
     public DateTime? ScheduledFor { get; set; }
     public string? Result { get; set; }
     public string? ReviewFeedback { get; set; }