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.

tests/ClaudeDo.Ui.Tests/ViewModels/WorktreesOverviewReconcileTickTests.cs

@@ -18,7 +18,7 @@ public class WorktreesOverviewReconcileTickTests
         public override Task<List<WorktreeOverviewDto>> GetWorktreesOverviewAsync(string? listId)
             => Task.FromResult(Worktrees.ToList());
         public override Task<MergeTargetsDto?> GetMergeTargetsAsync(string taskId)
-            => Task.FromResult<MergeTargetsDto?>(new MergeTargetsDto("main", new[] { "main" }));
+            => Task.FromResult<MergeTargetsDto?>(new MergeTargetsDto("main", new[] { "main" }, "chore(list): merge t"));
     }
 
     private sealed class NoopMergeCoordinator : IMergeCoordinator