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/Islands/DetailsIslandViewModel.cs

@@ -1073,6 +1073,10 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
             var result = await _worker.ApproveReviewAsync(Task.Id, Merge.SelectedMergeTarget ?? "");
             if (!hasChildren && result?.Status == "conflict")
                 await _merge.ResolveConflictAsync(Task.Id, Merge.SelectedMergeTarget ?? "");
+            // The merge itself already landed; the verify command failed, so the task stayed
+            // out of Done. Surface that instead of silently looking like nothing happened.
+            else if (!hasChildren && result?.Status == "verify_failed" && ShowErrorAsync != null)
+                await ShowErrorAsync(result.ErrorMessage ?? Loc.T("vm.detailsIsland.verifyFailed"));
         }
         catch (Exception ex)
         {