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.Worker/Hub/WorkerHub.cs

@@ -380,6 +380,38 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
 
     public bool CancelTask(string taskId) => _queue.CancelTask(taskId);
 
+    // Mirrors ExternalMcpService.DeleteTask so a UI-initiated delete gets the same
+    // TryAdvanceParentAsync side effect — a direct-repo delete from the details pane
+    // used to skip it, permanently wedging a WaitingForChildren parent whose last
+    // child was deleted from there.
+    public async Task DeleteTask(string taskId, CancellationToken cancellationToken)
+    {
+        await using var ctx = await _dbFactory.CreateDbContextAsync(cancellationToken);
+        var repo = new TaskRepository(ctx);
+        var task = await repo.GetByIdAsync(taskId, cancellationToken)
+            ?? throw new HubException("task not found");
+        if (task.Status == TaskStatus.Running)
+            throw new HubException("Cannot delete a running task. Cancel it first.");
+
+        try
+        {
+            await repo.DeleteAsync(taskId, cancellationToken);
+        }
+        // TaskRepository.DeleteAsync uses ExecuteDeleteAsync, which bypasses SaveChanges and
+        // surfaces provider errors directly as SqliteException rather than DbUpdateException.
+        catch (Exception ex) when (
+            (ex is Microsoft.Data.Sqlite.SqliteException || ex.InnerException is Microsoft.Data.Sqlite.SqliteException)
+            && (ex.Message.Contains("FOREIGN KEY", StringComparison.OrdinalIgnoreCase)
+                || ex.InnerException?.Message.Contains("FOREIGN KEY", StringComparison.OrdinalIgnoreCase) == true))
+        {
+            throw new HubException("This task has child tasks. Discard the planning session or delete child tasks first.");
+        }
+
+        if (task.ParentTaskId is not null)
+            await _state.TryAdvanceParentAsync(task.ParentTaskId);
+        await _broadcaster.TaskUpdated(taskId);
+    }
+
     public void WakeQueue() => _waker.Wake();
 
     public async Task<List<AgentInfo>> GetAgents() => await _agentService.ScanAsync();