feat(worker): add generic OperationProgress channel, port MergeProgress onto it

Merge/verify phases now broadcast over a generic (opKey, phase, current,
total) Hub event instead of a merge-specific one, so future producers
(worktree cleanup, startup recovery, planning integration) can reuse it.
IWorkerClient.MergeProgressEvent stays as a thin forwarder for existing
consumers (elapsed seconds riding in the generic "current" slot).

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -64,6 +64,7 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
 
     public event Action<UsageSnapshotDto>? UsageUpdatedEvent;
 
+    public event Action<string, string, int, int>? OperationProgressEvent;
     public event Action<string, string, int>? MergeProgressEvent;
 
     public event Action<string, string>? PlanningMergeStartedEvent;
@@ -174,9 +175,13 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
                 WorkerLogReceivedEvent?.Invoke(new WorkerLogEntry(message, level, timestampUtc)));
         });
 
-        _hub.On<string, string, int>("MergeProgress", (taskId, phase, elapsedSeconds) =>
+        _hub.On<string, string, int, int>("OperationProgress", (opKey, phase, current, total) =>
         {
-            Dispatcher.UIThread.Post(() => MergeProgressEvent?.Invoke(taskId, phase, elapsedSeconds));
+            Dispatcher.UIThread.Post(() =>
+            {
+                OperationProgressEvent?.Invoke(opKey, phase, current, total);
+                MergeProgressEvent?.Invoke(opKey, phase, current);
+            });
         });
 
         _hub.On<string, string>("PlanningMergeStarted", (planningTaskId, targetBranch) =>