chore(ui): remove IWorkerClient.MergeProgressEvent forwarder

Group C of the operation-feedback batch is fully merged; the two consumers
(DetailsIslandViewModel.ApproveReviewAsync, MergeModalViewModel) now handle
the verifying phase from the generic OperationProgressEvent handler instead
of a separate forwarder event.

src/ClaudeDo.Ui/Services/Interfaces/IWorkerClient.cs

@@ -52,13 +52,6 @@ public interface IWorkerClient : INotifyPropertyChanged
     /// total.</summary>
     event Action<string, string, int, int>? OperationProgressEvent;
 
-    /// <summary>(taskId, phase, elapsedSeconds) — phase of an in-flight single-task merge
-    /// ("merging" | "verifying"). Fires while the MergeTask call itself is still pending, so the
-    /// waiting UI can show what it's blocked on; the verify phase re-fires every 30 s. A thin
-    /// forwarder over <see cref="OperationProgressEvent"/> kept for existing consumers (elapsed
-    /// seconds riding in the generic "current" slot).</summary>
-    event Action<string, string, int>? MergeProgressEvent;
-
     event Action<string, string>? PlanningMergeStartedEvent;
     event Action<string, string>? PlanningSubtaskMergedEvent;
     /// <summary>(planningTaskId, subtaskId, conflictedFiles, externallyDriven). externallyDriven