fix(worker): propagate unit-merge failures instead of reporting success

A child merge that came back blocked/verify_failed/untracked_collision during a
parent/children unit merge used to vanish: DrainAsync only logged it server-side,
PlanningMergeAborted carried no reason, and ApproveReview/review_task always
reported StatusMerged for a task with children regardless of the real outcome,
so a failed unit merge left the parent stuck with no visible error.

- PlanningMergeOrchestrator.StartAsync/ContinueAsync/DrainAsync now return a
  PlanningMergeResult(Status, Reason) instead of void, and PlanningMergeAborted
  carries that reason to the UI.
- WorkerHub.ApproveReview and ExternalMcpService.ReviewTask's approve branch
  propagate the real status/reason for a parent with children instead of
  hardcoding "merged" (or masking a non-conflict failure as "conflict").
- StartAsync now requires the parent to already be WaitingForReview for
  improvement parents too, not just planning ones, so a stale caller can no
  longer trigger a partial child merge.
- HasActiveMerge now also covers the window between the last child merging and
  FinalizeParentDoneAsync completing, closing a gap where a concurrent Cancel
  could race the parent's own approve-to-Done transition.
- IslandsShellViewModel.OnPlanningMergeAborted flashes the reason via
  FlashFooterError instead of only clearing the external-merge banner.

src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs

@@ -36,7 +36,7 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
     private readonly Action<string> _workerWorktreeUpdatedHandler;
     private readonly Action<string> _workerTaskUpdatedHandler;
     private readonly Action<string, string> _workerPlanningMergeStartedHandler;
-    private readonly Action<string> _workerPlanningMergeAbortedHandler;
+    private readonly Action<string, string?> _workerPlanningMergeAbortedHandler;
     private readonly Action<string> _workerPlanningCompletedHandler;
 
     [ObservableProperty] private bool _isNotesMode;
@@ -432,7 +432,7 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
         };
         _worker.PlanningMergeStartedEvent += _workerPlanningMergeStartedHandler;
 
-        _workerPlanningMergeAbortedHandler = planningTaskId =>
+        _workerPlanningMergeAbortedHandler = (planningTaskId, _) =>
         {
             if (Task?.Id == planningTaskId) IsMergeDraining = false;
         };