feat(worker): add post-merge verification gate for list merges

Per-list optional VerifyCommand (list_config.verify_command) runs via
VerifyCommandRunner in the list's working dir right after a successful
merge/continue-merge, before the task is allowed to reach Done. A
non-zero exit or timeout leaves the merge in place but keeps the task
out of Done and reports StatusVerifyFailed with an output excerpt
through MergeResultDto/review_task; no command configured behaves
exactly as before. Merges against the same repo are now serialized
per working dir so a running verify can't be interrupted by a second
merge landing mid-build. Adds the field to the List Settings modal
(en/de localized) and covers success/failure/timeout in
TaskMergeServiceTests + VerifyCommandRunnerTests.

src/ClaudeDo.Ui/ViewModels/Modals/ListSettingsModalViewModel.cs

@@ -30,6 +30,9 @@ public sealed partial class ListSettingsModalViewModel : ViewModelBase
     [ObservableProperty] private string _defaultCommitType = CommitTypeRegistry.DefaultType;
     // A manual list holds reminders: tasks created here start out manual (TaskEntity.IsManual).
     [ObservableProperty] private bool _isManual;
+    // Optional post-merge verification command (build/test), run in WorkingDir after a merge
+    // lands; a non-zero exit keeps the task out of Done instead of silently reporting merged.
+    [ObservableProperty] private string _verifyCommand = "";
 
     public ObservableCollection<string> CommitTypeOptions { get; } = new(CommitTypeRegistry.Types);
 
@@ -61,6 +64,8 @@ public sealed partial class ListSettingsModalViewModel : ViewModelBase
         DefaultCommitType = string.IsNullOrWhiteSpace(defaultCommitType) ? CommitTypeRegistry.DefaultType : defaultCommitType;
 
         await Agent.LoadForListAsync(listId, ct);
+        var cfg = await _worker.GetListConfigAsync(listId);
+        VerifyCommand = cfg?.VerifyCommand ?? "";
     }
 
     [RelayCommand]
@@ -73,7 +78,7 @@ public sealed partial class ListSettingsModalViewModel : ViewModelBase
             DefaultCommitType,
             IsManual));
 
-        await Agent.SaveAsync();
+        await Agent.SaveAsync(string.IsNullOrWhiteSpace(VerifyCommand) ? null : VerifyCommand);
 
         CloseAction?.Invoke();
     }