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.Worker/External/ExternalMcpService.cs

@@ -703,16 +703,21 @@ public sealed class ExternalMcpService
                 // externallyDriven: true — this call came from an MCP session, not the UI's
                 // Approve button. A unit-merge conflict must not auto-open the in-app resolver;
                 // the driving session resolves it via continue_merge/abort_merge instead.
-                await _planningMerge.StartAsync(taskId, targetBranch ?? "", cancellationToken, externallyDriven: true, progress);
-                var parentDone = (await _tasks.GetByIdAsync(taskId, cancellationToken))!.Status == TaskStatus.Done;
-                mergeStatus = parentDone ? TaskMergeService.StatusMerged : TaskMergeService.StatusConflict;
-                if (!parentDone)
+                var startResult = await _planningMerge.StartAsync(taskId, targetBranch ?? "", cancellationToken, externallyDriven: true, progress);
+                if (startResult.Status == TaskMergeService.StatusBlocked)
+                    throw new InvalidOperationException(startResult.Reason ?? "approve failed");
+                mergeStatus = startResult.Status;
+                if (startResult.Status == TaskMergeService.StatusConflict)
                 {
                     var list = await _lists.GetByIdAsync(task.ListId, cancellationToken);
                     repoPath = list?.WorkingDir;
                     mergeMessage = "unit merge paused on a conflict — markers left in the working tree; " +
                         "resolve them then call continue_merge with the parent task id, or abort_merge to cancel";
                 }
+                else if (startResult.Status != TaskMergeService.StatusMerged)
+                {
+                    mergeMessage = startResult.Reason;
+                }
             }
             else
             {