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/Runner/CommitMessageBuilder.cs

@@ -1,5 +1,6 @@
 using System.Text;
 using System.Text.RegularExpressions;
+using ClaudeDo.Data.Models;
 
 namespace ClaudeDo.Worker.Runner;
 
@@ -28,6 +29,23 @@ public static class CommitMessageBuilder
         return sb.ToString();
     }
 
+    /// <summary>
+    /// Message for the merge commit that lands a task branch. Same Conventional-Commits header
+    /// shape as the task's own commits (<see cref="Build"/>) with an explicit `merge ` verb, so
+    /// merge commits stay parseable *and* greppable, plus the task-id trailer. A blank commit
+    /// type falls back to <see cref="CommitTypeRegistry.DefaultType"/>; a list name that slugs
+    /// to nothing drops the scope rather than emitting an empty `()`.
+    /// </summary>
+    public static string BuildMerge(string commitType, string listName, string taskTitle, string taskId)
+    {
+        var type = string.IsNullOrWhiteSpace(commitType) ? CommitTypeRegistry.DefaultType : commitType.Trim();
+        var slug = ToSlug(listName);
+        var scope = slug.Length == 0 ? "" : $"({slug})";
+        var title = Truncate(taskTitle.Trim(), 60);
+
+        return $"{type}{scope}: merge {title}\n\nClaudeDo-Task: {taskId}";
+    }
+
     public static string ToSlug(string name)
     {
         var lower = name.ToLowerInvariant();