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.

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -128,7 +128,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
         var factory = _db.CreateFactory();
         var maintenance = new WorktreeMaintenanceService(factory, git, NullLogger<WorktreeMaintenanceService>.Instance);
         var state = TaskStateServiceBuilder.Build(factory).State;
-        var merge = new TaskMergeService(factory, git, _broadcaster, state, NullLogger<TaskMergeService>.Instance);
+        var merge = new TaskMergeService(factory, git, _broadcaster, state, new VerifyCommandRunner(), NullLogger<TaskMergeService>.Instance);
         var aggregator = new PlanningAggregator(factory, git, NullLogger<PlanningAggregator>.Instance);
         var planningMerge = new PlanningMergeOrchestrator(
             factory, merge, aggregator, _broadcaster, git, state, NullLogger<PlanningMergeOrchestrator>.Instance);