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/Planning/Interfaces/IActiveMergeState.cs

@@ -7,4 +7,9 @@ namespace ClaudeDo.Worker.Planning;
 public interface IActiveMergeState
 {
     bool HasActiveMerge(string taskId);
+
+    // In-memory state above is empty after a worker restart. This falls back to an on-disk
+    // check (repo mid-merge) for the one case that matters: a unit-merge parent paused on a
+    // conflict when the process died mid-drain.
+    Task<bool> HasActiveMergeAsync(string taskId, CancellationToken ct);
 }