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/IslandsShellViewModel.cs

@@ -218,7 +218,14 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
         _ = OpenPlanningConflictAsync(planningTaskId, subtaskId);
     }
 
-    public void OnPlanningMergeAborted(string planningTaskId) => ClearExternalMergeConflict(planningTaskId);
+    public void OnPlanningMergeAborted(string planningTaskId, string? reason = null)
+    {
+        ClearExternalMergeConflict(planningTaskId);
+        // A deliberate abort/conflict pause and a real merge failure both flow through this
+        // event; only the latter carries a reason worth surfacing in the footer strip.
+        if (!string.IsNullOrWhiteSpace(reason))
+            FlashFooterError(reason);
+    }
     public void OnPlanningMergeCompleted(string planningTaskId) => ClearExternalMergeConflict(planningTaskId);
 
     private void ClearExternalMergeConflict(string planningTaskId)