fix(worker): advance parent when the last non-terminal child is deleted

DeleteTask never re-evaluated a WaitingForChildren parent, so deleting
the last non-terminal child left it stuck (the BlockedByTaskId SET NULL
FK only repairs the blocked chain, not parent status). Expose
TaskStateService.TryAdvanceParentAsync on the interface and call it
from ExternalMcpService.DeleteTask after a child delete.

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -357,6 +357,8 @@ public sealed class ExternalMcpService
             throw new InvalidOperationException("Cannot delete a running task. Cancel it first.");
 
         await _tasks.DeleteAsync(taskId, cancellationToken);
+        if (task.ParentTaskId is not null)
+            await _state.TryAdvanceParentAsync(task.ParentTaskId);
         await _broadcaster.TaskUpdated(taskId);
         return new DeleteTaskResult(true, taskId);
     }

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

@@ -24,5 +24,10 @@ public interface ITaskStateService
     Task<TransitionResult> BlockOnAsync(string taskId, string predecessorTaskId, CancellationToken ct);
     Task<TransitionResult> UnblockAsync(string taskId, CancellationToken ct);
 
+    // Surfaces a WaitingForChildren parent for review once all its children are terminal.
+    // Best-effort (swallows and logs failures) — safe to call after any child mutation,
+    // e.g. deleting the last non-terminal child (no terminal transition fires for a delete).
+    Task TryAdvanceParentAsync(string parentId);
+
     Task<int> RecoverStaleRunningAsync(string reason, CancellationToken ct);
 }

src/ClaudeDo.Worker/State/TaskStateService.cs

@@ -395,9 +395,18 @@ public sealed class TaskStateService : ITaskStateService
             _logger.LogWarning(ex, "PlanningChain advance failed for {TaskId}", taskId);
         }
 
+        await TryAdvanceParentAsync(parentId);
+    }
+
+    // Any parent (planning or improvement) sitting in WaitingForChildren surfaces for review
+    // once every child is terminal (Done/Failed/Cancelled). A failed or cancelled child does
+    // not wedge the parent — it is flagged on the result. Also called directly after a child
+    // is deleted, since no terminal transition fires in that case.
+    public async Task TryAdvanceParentAsync(string parentId)
+    {
         try
         {
-            await TryAdvanceParentAsync(parentId);
+            await AdvanceParentIfAllChildrenTerminalAsync(parentId);
         }
         catch (Exception ex)
         {
@@ -405,10 +414,7 @@ public sealed class TaskStateService : ITaskStateService
         }
     }
 
-    // Any parent (planning or improvement) sitting in WaitingForChildren surfaces for review
-    // once every child is terminal (Done/Failed/Cancelled). A failed or cancelled child does
-    // not wedge the parent — it is flagged on the result.
-    private async Task TryAdvanceParentAsync(string parentId)
+    private async Task AdvanceParentIfAllChildrenTerminalAsync(string parentId)
     {
         string? parentResult;
         List<TaskStatus> childStatuses;