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.

tests/ClaudeDo.Worker.Tests/UiVm/TasksIslandViewModelPlanningTests.cs

@@ -118,7 +118,7 @@ sealed class FakeWorkerClient : IWorkerClient
     public event Action<string, string>? PlanningMergeStartedEvent;
     public event Action<string, string>? PlanningSubtaskMergedEvent;
     public event Action<string, string, IReadOnlyList<string>, bool>? PlanningMergeConflictEvent;
-    public event Action<string>? PlanningMergeAbortedEvent;
+    public event Action<string, string?>? PlanningMergeAbortedEvent;
     public event Action<string>? PlanningCompletedEvent;
     public event Action<PrimeFiredEvent>? PrimeFired;
     public event Action<UsageSnapshotDto>? UsageUpdatedEvent;