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.

src/ClaudeDo.Worker/State/TaskStateService.cs

@@ -286,7 +286,7 @@ public sealed class TaskStateService : ITaskStateService
         // from under it would flip the parent to Cancelled while the orchestrator keeps merging —
         // FinalizeParentDoneAsync then finds the parent no longer WaitingForReview and gives up,
         // leaving the merged children's diffs stranded with no way to roll them back.
-        if (_mergeState().HasActiveMerge(taskId))
+        if (await _mergeState().HasActiveMergeAsync(taskId, ct))
             return new TransitionResult(false, "A merge is in progress for this task; wait for it to finish before cancelling.");
 
         List<string> cancelledChildIds;