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/State/TaskStateService.cs

@@ -505,8 +505,8 @@ public sealed class TaskStateService : ITaskStateService
                 .Select(t => t.Status)
                 .ToListAsync(CancellationToken.None);
         }
-        if (childStatuses.Count == 0) return;
-
+        // No early-out on an empty list: zero children left (e.g. the last one was just
+        // deleted) counts as "all terminal" — .All() on an empty sequence is vacuously true.
         bool allTerminal = childStatuses.All(s =>
             s == TaskStatus.Done || s == TaskStatus.Failed || s == TaskStatus.Cancelled);
         if (!allTerminal) return;