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/WorktreesOverviewModalViewModel.cs

@@ -440,8 +440,8 @@ public sealed partial class WorktreesOverviewModalViewModel : ViewModelBase
                 MergeResultDto result;
                 try
                 {
-                    result = await mergeFn(row.TaskId, target!, false,
-                        Loc.T("vm.merge.commitMessage", row.TaskTitle));
+                    // Blank message: the worker builds the conventional default per task.
+                    result = await mergeFn(row.TaskId, target!, false, "");
                 }
                 catch
                 {