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/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);
 }