fix(merge): conventional merge-commit default and live verify progress

The merge commit message was hand-rolled per caller ("Merge task: <title>",
"Merge <branch>", "Merge subtask") and ignored the task's commit type. Every
caller now passes a blank message and TaskMergeService fills in
CommitMessageBuilder.BuildMerge -> {commitType}(list-slug): merge <title> plus the
ClaudeDo-Task trailer; the merge modal prefills it from GetMergeTargets.

A merge whose list has a verify command holds the MergeTask call for minutes (5m46s
on this repo), during which the modal only disabled its button - no spinner, no
message, so a landed merge looked like a dead app. TaskMergeService now broadcasts
MergeProgress(taskId, phase, elapsedSeconds) for the merging and verifying phases
(re-reported every 30s) plus a WorkerLog line when verify starts; the modal shows a
spinner and the localized phase.

src/ClaudeDo.Ui/ViewModels/Modals/MergeModalViewModel.cs

@@ -21,6 +21,12 @@ public sealed partial class MergeModalViewModel : ViewModelBase
     [ObservableProperty] private string _commitMessage = "";
 
     [ObservableProperty][NotifyCanExecuteChangedFor(nameof(SubmitCommand))] private bool _isBusy;
+
+    /// What the pending merge is doing right now, fed by the worker's MergeProgress broadcast.
+    /// A merge whose list has a verify command can occupy this call for minutes — without this the
+    /// modal only greys the button out and looks dead.
+    [ObservableProperty] private string? _progressMessage;
+
     [ObservableProperty] private string? _errorMessage;
     [ObservableProperty] private string? _warningMessage;
     [ObservableProperty] private string? _successMessage;
@@ -58,6 +64,10 @@ public sealed partial class MergeModalViewModel : ViewModelBase
                 ErrorMessage = Loc.T("vm.merge.workerOfflineBranches");
                 return;
             }
+            // The worker owns the default message — only it knows the task's commit type and the
+            // list name the scope is slugged from. The locale string stays as the offline fallback.
+            if (!string.IsNullOrWhiteSpace(targets.DefaultCommitMessage))
+                CommitMessage = targets.DefaultCommitMessage;
             foreach (var b in targets.LocalBranches) Branches.Add(b);
             SelectedBranch = Branches.Contains(targets.DefaultBranch)
                 ? targets.DefaultBranch
@@ -81,6 +91,10 @@ public sealed partial class MergeModalViewModel : ViewModelBase
         ErrorMessage = null;
         WarningMessage = null;
         SuccessMessage = null;
+        // Subscribed only for the duration of the call: the worker's broadcast reaches every
+        // client, and a transient VM left on that event would outlive its window.
+        ProgressMessage = Loc.T("vm.merge.progressMerging");
+        _worker.MergeProgressEvent += OnMergeProgress;
         try
         {
             var result = await _worker.MergeTaskAsync(
@@ -133,10 +147,28 @@ public sealed partial class MergeModalViewModel : ViewModelBase
         }
         finally
         {
+            _worker.MergeProgressEvent -= OnMergeProgress;
+            ProgressMessage = null;
             IsBusy = false;
         }
     }
 
+    private void OnMergeProgress(string taskId, string phase, int elapsedSeconds)
+    {
+        if (taskId != TaskId) return;
+        ProgressMessage = phase switch
+        {
+            MergePhaseVerifying => Loc.T("vm.merge.progressVerifying", FormatElapsed(elapsedSeconds)),
+            _ => Loc.T("vm.merge.progressMerging"),
+        };
+    }
+
+    /// Mirrors TaskMergeService.PhaseVerifying — a hub payload token, not a display string.
+    private const string MergePhaseVerifying = "verifying";
+
+    private static string FormatElapsed(int seconds) =>
+        TimeSpan.FromSeconds(Math.Max(0, seconds)).ToString(@"mm\:ss");
+
     [RelayCommand]
     private void Cancel() => CloseAction?.Invoke();
 }