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.Ui.Tests/IslandsShellViewModelExternalMergeTests.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Threading.Tasks;
+using ClaudeDo.Data.Models;
 using ClaudeDo.Ui.ViewModels;
 using Xunit;
 
@@ -43,6 +44,31 @@ public class IslandsShellViewModelExternalMergeTests
         Assert.False(vm.IsExternalMergeBannerVisible);
     }
 
+    // Covers the unit-merge-error-propagation fix: a real merge failure (blocked/verify_failed/
+    // untracked_collision) reaches the UI as a reason on PlanningMergeAborted, and must surface
+    // in the footer strip rather than silently vanishing once the banner is cleared.
+    [Fact]
+    public void Aborted_WithReason_FlashesFooterError()
+    {
+        var vm = new IslandsShellViewModel();
+
+        vm.OnPlanningMergeAborted("plan1", "child merge blocked: unrelated histories");
+
+        Assert.Equal(WorkerLogLevel.Error, vm.WorkerLogLevel);
+        Assert.Contains("child merge blocked: unrelated histories", vm.WorkerLogText);
+        Assert.True(vm.IsWorkerLogVisible);
+    }
+
+    [Fact]
+    public void Aborted_WithoutReason_DoesNotFlashFooterError()
+    {
+        var vm = new IslandsShellViewModel();
+
+        vm.OnPlanningMergeAborted("plan1", null);
+
+        Assert.False(vm.IsWorkerLogVisible);
+    }
+
     [Fact]
     public void Completed_ClearsBanner()
     {