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);
     }