feat(worker): allow update_task_status to set Cancelled

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -262,9 +262,10 @@ public sealed class ExternalMcpService
     }
 
     [McpServerTool, Description(
-        "Update a task's status. Only 'Idle' and 'Queued' are permitted externally — " +
-        "use run_task_now or cancel_task for execution control, and review_task to act on a WaitingForReview task. " +
-        "Settable: Idle (reset to editable), Queued (enqueue for execution). " +
+        "Update a task's status. Only 'Idle', 'Queued' and 'Cancelled' are permitted externally — " +
+        "use run_task_now for execution control, and review_task to act on a WaitingForReview task. " +
+        "Settable: Idle (reset to editable), Queued (enqueue for execution), " +
+        "Cancelled (retire the task without deleting it; it can be reset to Idle later). " +
         "Full lifecycle: Idle → Queued → Running → WaitingForReview → Done | Failed | Cancelled.")]
     public async Task<TaskDto> UpdateTaskStatus(
         string taskId,
@@ -291,9 +292,15 @@ public sealed class ExternalMcpService
                     throw new InvalidOperationException(enqueueResult.Reason ?? "Cannot enqueue task.");
                 break;
 
+            case TaskStatus.Cancelled:
+                var cancelResult = await _state.CancelAsync(taskId, DateTime.UtcNow, cancellationToken, allowFromIdle: true);
+                if (!cancelResult.Ok)
+                    throw new InvalidOperationException(cancelResult.Reason ?? "Cannot cancel task.");
+                break;
+
             default:
                 throw new InvalidOperationException(
-                    $"Status '{target}' is not settable externally. Use run_task_now or cancel_task.");
+                    $"Status '{target}' is not settable externally. Use run_task_now or review_task.");
         }
 
         var reload = (await _tasks.GetByIdAsync(taskId, cancellationToken))!;

src/ClaudeDo.Worker/State/Interfaces/ITaskStateService.cs

@@ -9,7 +9,7 @@ public interface ITaskStateService
     Task<TransitionResult> SubmitInteractiveForReviewAsync(string taskId, DateTime finishedAt, CancellationToken ct);
     Task<TransitionResult> SubmitForChildrenAsync(string taskId, DateTime finishedAt, string? result, CancellationToken ct);
     Task<TransitionResult> FailAsync(string taskId, DateTime finishedAt, string? error, CancellationToken ct);
-    Task<TransitionResult> CancelAsync(string taskId, DateTime finishedAt, CancellationToken ct);
+    Task<TransitionResult> CancelAsync(string taskId, DateTime finishedAt, CancellationToken ct, bool allowFromIdle = false);
     Task<TransitionResult> ResetToIdleAsync(string taskId, CancellationToken ct);
 
     Task<TransitionResult> ApproveReviewAsync(string taskId, CancellationToken ct);

src/ClaudeDo.Worker/State/TaskStateService.cs

@@ -241,7 +241,13 @@ public sealed class TaskStateService : ITaskStateService
         return new TransitionResult(true, null);
     }
 
-    public async Task<TransitionResult> CancelAsync(string taskId, DateTime finishedAt, CancellationToken ct)
+    // allowFromIdle: only the external update_task_status(Cancelled) path sets this — it lets
+    // an Idle task be retired without deleting it. Every other caller (hub CancelReview,
+    // PlanningChainCoordinator's chain-walk, batch cancel) relies on Idle staying a no-op here;
+    // PlanningChainCoordinator specifically uses "parked back to Idle" as a deliberate opt-out
+    // signal, so do not flip this default.
+    public async Task<TransitionResult> CancelAsync(
+        string taskId, DateTime finishedAt, CancellationToken ct, bool allowFromIdle = false)
     {
         List<string> cancelledChildIds;
         await using (var ctx = await _dbFactory.CreateDbContextAsync(ct))
@@ -250,7 +256,8 @@ public sealed class TaskStateService : ITaskStateService
                 .Where(t => t.Id == taskId &&
                     (t.Status == TaskStatus.Running || t.Status == TaskStatus.Queued
                      || t.Status == TaskStatus.WaitingForReview
-                     || t.Status == TaskStatus.WaitingForChildren))
+                     || t.Status == TaskStatus.WaitingForChildren
+                     || (allowFromIdle && t.Status == TaskStatus.Idle)))
                 .ExecuteUpdateAsync(s => s
                     .SetProperty(t => t.Status, TaskStatus.Cancelled)
                     .SetProperty(t => t.FinishedAt, finishedAt), ct);