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/Hub/HubBroadcaster.cs

@@ -46,6 +46,11 @@ public sealed class HubBroadcaster : IPrimeBroadcaster, IRefineBroadcaster
     public Task WorkerLog(string message, WorkerLogLevel level, DateTime timestampUtc) =>
         _hub.Clients.All.SendAsync("WorkerLog", message, level, timestampUtc);
 
+    // Phase of an in-flight single-task merge (see TaskMergeService.Phase*), so a client waiting
+    // on the MergeTask call can show what it is waiting for instead of a frozen button.
+    public Task MergeProgress(string taskId, string phase, int elapsedSeconds) =>
+        _hub.Clients.All.SendAsync("MergeProgress", taskId, phase, elapsedSeconds);
+
     public Task PlanningMergeStarted(string planningTaskId, string targetBranch) =>
         _hub.Clients.All.SendAsync("PlanningMergeStarted", planningTaskId, targetBranch);