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/Planning/PlanningMergeOrchestrator.cs

@@ -227,7 +227,7 @@ public sealed class PlanningMergeOrchestrator : IActiveMergeState
                     subtaskId,
                     state.TargetBranch,
                     removeWorktree: true,
-                    commitMessage: "Merge subtask",
+                    commitMessage: "",   // blank -> TaskMergeService builds the conventional default
                     leaveConflictsInTree: true,
                     ct);