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.Worker/Lifecycle/Interfaces/IVerifyCommandRunner.cs

@@ -0,0 +1,8 @@
+namespace ClaudeDo.Worker.Lifecycle;
+
+public sealed record VerifyCommandResult(int ExitCode, bool TimedOut, string Output);
+
+public interface IVerifyCommandRunner
+{
+    Task<VerifyCommandResult> RunAsync(string workingDir, string command, TimeSpan timeout, CancellationToken ct);
+}