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).

tests/ClaudeDo.Worker.Tests/UiVm/TasksIslandViewModelPlanningTests.cs

@@ -113,6 +113,7 @@ sealed class FakeWorkerClient : IWorkerClient
     public event Action<bool>? PrepFinishedEvent;
     public event Action<string>? RefineStartedEvent;
     public event Action<string, bool, string?>? RefineFinishedEvent;
+    public event Action<string, string, int, int>? OperationProgressEvent;
     public event Action<string, string, int>? MergeProgressEvent;
     public event Action<string, string>? PlanningMergeStartedEvent;
     public event Action<string, string>? PlanningSubtaskMergedEvent;