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.Ui.Tests/StubWorkerClient.cs

@@ -31,6 +31,7 @@ public abstract class StubWorkerClient : 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;
@@ -56,6 +57,7 @@ public abstract class StubWorkerClient : IWorkerClient
     public void RaiseHandoffRequested(string taskId, IReadOnlyList<string> survivingTaskIds, string nextPhase = "wait") => HandoffRequestedEvent?.Invoke(taskId, survivingTaskIds, nextPhase);
     public void RaisePlanningMergeConflict(string planningTaskId, string subtaskId, IReadOnlyList<string> files, bool externallyDriven)
         => PlanningMergeConflictEvent?.Invoke(planningTaskId, subtaskId, files, externallyDriven);
+    public void RaiseOperationProgress(string opKey, string phase, int current, int total) => OperationProgressEvent?.Invoke(opKey, phase, current, total);
     public void RaiseMergeProgress(string taskId, string phase, int elapsedSeconds) => MergeProgressEvent?.Invoke(taskId, phase, elapsedSeconds);
     public void RaisePlanningMergeStarted(string planningTaskId, string targetBranch) => PlanningMergeStartedEvent?.Invoke(planningTaskId, targetBranch);
     public void RaisePlanningMergeAborted(string planningTaskId) => PlanningMergeAbortedEvent?.Invoke(planningTaskId);

tests/ClaudeDo.Worker.Tests/Services/TaskMergeServiceTests.cs

@@ -335,9 +335,9 @@ public class TaskMergeServiceTests : IDisposable
             commitMessage: "", ct: CancellationToken.None);
 
         Assert.Equal("merged", result.Status);
-        Assert.Contains(proxy.Calls, c => c.Method == "MergeProgress"
+        Assert.Contains(proxy.Calls, c => c.Method == "OperationProgress"
             && (string?)c.Args[0] == task.Id && (string?)c.Args[1] == TaskMergeService.PhaseMerging);
-        Assert.Contains(proxy.Calls, c => c.Method == "MergeProgress"
+        Assert.Contains(proxy.Calls, c => c.Method == "OperationProgress"
             && (string?)c.Args[0] == task.Id && (string?)c.Args[1] == TaskMergeService.PhaseVerifying);
         Assert.Contains(proxy.Calls, c => c.Method == "WorkerLog"
             && c.Args[0] is string s && s.Contains("Verify command running"));

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;