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.Data/Models/ListConfigEntity.cs

@@ -8,6 +8,7 @@ public sealed class ListConfigEntity
     public string? AgentPath { get; set; }
     public int? MaxTurns { get; set; }
     public string? SessionSkills { get; set; }
+    public string? VerifyCommand { get; set; }
 
     // Navigation property
     public ListEntity List { get; set; } = null!;