fix(worker): survive a worker restart when cancelling a mid-merge unit-merge parent

CancelAsync only checked PlanningMergeOrchestrator's in-memory HasActiveMerge, which is
empty after a restart. IActiveMergeState gains an async fallback that checks on-disk
(GitService.IsMidMergeAsync) for a WaitingForReview parent with children, so the guard
still blocks cancel until the merge is continued or aborted.

tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs

@@ -162,6 +162,36 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
         Assert.Contains(spy, c => c.Method == "PlanningCompleted");
     }
 
+    [Fact]
+    public async Task CancelAsync_UnitMergeParentMidMergeAfterSimulatedRestart_BlocksUntilAborted()
+    {
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        var (parentId, _, _, _) = await SeedPlanningThreeChildrenMiddleConflictsAsync(db, repo);
+
+        var (orch, _) = BuildOrchestrator(db);
+        var startResult = await orch.StartAsync(parentId, "main", CancellationToken.None);
+        Assert.Equal(TaskMergeService.StatusConflict, startResult.Status);
+
+        // Simulate a worker restart: a brand-new orchestrator instance has an empty in-memory
+        // _states dictionary, so the guard must fall back to the on-disk mid-merge check.
+        var (freshOrch, _) = BuildOrchestrator(db);
+        var cancelBuilt = TaskStateServiceBuilder.Build(db.CreateFactory(), () => freshOrch);
+
+        var blocked = await cancelBuilt.State.CancelAsync(parentId, DateTime.UtcNow, CancellationToken.None);
+        Assert.False(blocked.Ok);
+
+        using (var ctx = db.CreateContext())
+            Assert.Equal(TaskStatus.WaitingForReview, ctx.Tasks.Single(t => t.Id == parentId).Status);
+
+        GitRepoFixture.RunGit(repo.RepoDir, "merge", "--abort");
+
+        var ok = await cancelBuilt.State.CancelAsync(parentId, DateTime.UtcNow, CancellationToken.None);
+        Assert.True(ok.Ok);
+    }
+
     private async Task<(string parentId, string subA, string subB, string subC)> SeedPlanningThreeChildrenMiddleConflictsAsync(
         DbFixture db, GitRepoFixture repo)
     {