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/DetailsIslandPlanningTests.cs

@@ -129,7 +129,7 @@ public class DetailsIslandPlanningTests : IDisposable
 
         var fake = new FakeWorkerClient
         {
-            MergeTargetsResult = new MergeTargetsDto("main", new[] { "main", "dev" }),
+            MergeTargetsResult = new MergeTargetsDto("main", new[] { "main", "dev" }, "chore(list): merge t"),
         };
 
         var vm = BuildVm(fake);