fix(worker,ui): route details-pane task delete through worker to advance blocked parents

Deleting a child from the details pane hard-deleted straight from the UI
process via TaskRepository, bypassing TaskStateService.TryAdvanceParentAsync
entirely. Deleting the last child of a WaitingForChildren parent that way
left it wedged forever. WorkerHub.DeleteTask now mirrors the MCP delete_task
tool (running-task guard, FK-friendly error, advance-parent call), and the
UI goes through it.

TryAdvanceParentAsync also short-circuited when zero children remained,
treating "no children left" as "nothing to evaluate" instead of "all done" -
removed the early return so an empty child list (vacuously) counts as all
terminal.

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -327,6 +327,19 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
         await _hub.InvokeAsync("CancelTask", taskId);
     }
 
+    public async Task<(bool Ok, string? Error)> DeleteTaskAsync(string taskId)
+    {
+        try
+        {
+            await _hub.InvokeAsync("DeleteTask", taskId, CancellationToken.None);
+            return (true, null);
+        }
+        catch (HubException ex)
+        {
+            return (false, ex.Message);
+        }
+    }
+
     public async Task WakeQueueAsync()
     {
         await _hub.InvokeAsync("WakeQueue");