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/State/TaskStateServiceTests.cs

@@ -15,6 +15,7 @@ file sealed class FakeActiveMergeState : IActiveMergeState
 {
     public HashSet<string> ActiveTaskIds { get; } = new();
     public bool HasActiveMerge(string taskId) => ActiveTaskIds.Contains(taskId);
+    public Task<bool> HasActiveMergeAsync(string taskId, CancellationToken ct) => Task.FromResult(HasActiveMerge(taskId));
 }
 
 public sealed class TaskStateServiceTests : IDisposable