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.Worker/External/ExternalMcpService.cs

@@ -997,9 +997,9 @@ public sealed class ExternalMcpService
             return new MergeTaskResultDto(false, null, []);
         }
 
-        var commitMessage = $"Merge task branch for: {task.Title}";
+        // Blank on purpose: TaskMergeService builds the conventional default message.
         var result = await _merge.MergeAsync(
-            taskId, targetBranch, removeWorktree: false, commitMessage, leaveConflictsInTree, cancellationToken, progress);
+            taskId, targetBranch, removeWorktree: false, commitMessage: "", leaveConflictsInTree, cancellationToken, progress);
 
         if (result.Status == TaskMergeService.StatusMerged)
         {