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/Interfaces/IWorkerClient.cs

@@ -34,9 +34,18 @@ public interface IWorkerClient : INotifyPropertyChanged
     event Action<string>? PrepLineEvent;
     event Action<bool>? PrepFinishedEvent;
 
+    /// <summary>(opKey, phase, current, total) — generic progress channel for long-running worker
+    /// operations (merge phases, worktree cleanup, startup recovery, planning integration, ...).
+    /// opKey is the TaskId for a task-bound operation, otherwise a stable string. current/total
+    /// is meant to be shown as text, never a progress bar — most operations have no meaningful
+    /// 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.</summary>
+    /// 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;