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.

tests/ClaudeDo.Ui.Tests/StubWorkerClient.cs

@@ -79,6 +79,7 @@ public abstract class StubWorkerClient : IWorkerClient
     public virtual Task<PendingQuestionDto?> GetPendingQuestionAsync(string taskId) => Task.FromResult(PendingQuestion);
     public virtual Task ResetTaskAsync(string taskId) => Task.CompletedTask;
     public virtual Task CancelTaskAsync(string taskId) => Task.CompletedTask;
+    public virtual Task<(bool Ok, string? Error)> DeleteTaskAsync(string taskId) => Task.FromResult<(bool, string?)>((true, null));
     public virtual Task<List<AgentInfo>> GetAgentsAsync() => Task.FromResult(new List<AgentInfo>());
     public virtual Task RefreshAgentsAsync() => Task.CompletedTask;
     public virtual Task<SeedResultDto?> RestoreDefaultAgentsAsync() => Task.FromResult<SeedResultDto?>(null);