feat(worker): chain a normal parent's children on WaitingForChildren

A standalone parent (PlanningPhase.None) with existing Idle/Queued children
now runs itself first, then sequences its children instead of flat-enqueuing
all of them at once: SubmitForChildrenAsync calls
PlanningChainCoordinator.SetupChainAsync(enqueue: true) right after the
Running -> WaitingForChildren transition, mirroring the planning path but
firing immediately instead of waiting for an explicit "Queue plan". Planning
parents are left untouched (they queue via QueuePlanAsync), and a child
already Running makes SetupChainAsync throw, which is now caught and logged
so it can't fail the parent's own transition.

src/ClaudeDo.Worker/Runner/TaskRunner.cs

@@ -553,8 +553,6 @@ public sealed class TaskRunner
             var transition = await _state.SubmitForChildrenAsync(task.Id, finishedAt, reviewResult, CancellationToken.None);
             if (!transition.Ok)
                 _logger.LogWarning("SubmitForChildrenAsync rejected for task {TaskId}: {Reason}", task.Id, transition.Reason);
-            foreach (var child in pendingChildren)
-                await _state.EnqueueAsync(child.Id, CancellationToken.None);
             await _broadcaster.WorkerLog(
                 $"Finished #{task.Number} \"{task.Title}\" (waiting on {pendingChildren.Count} improvement(s))",
                 WorkerLogLevel.Success, DateTime.UtcNow);