feat(ui): warn when the running worker predates the selected repo's merged HEAD

Stamps ClaudeDo.Worker's build with its exact git SHA (SourceRevisionId ->
InformationalVersion) and exposes it via a new GetWorkerBuildInfo hub call.
For the currently selected list, the shell compares that SHA against the
list's git HEAD (GitService.IsAncestorAsync) and shows a persistent footer
banner -- never auto-clearing, never shown on an unrelated repo or when the
ancestry can't be determined -- so "verified against a merge" claims aren't
silently made against a stale process. No auto-restart; the banner just
offers the existing RestartWorkerCommand.

tests/ClaudeDo.Ui.Tests/StubWorkerClient.cs

@@ -150,6 +150,8 @@ public abstract class StubWorkerClient : IWorkerClient
     public virtual Task DeleteDailyNoteAsync(string id) => Task.CompletedTask;
     public string LastPrepLog = "";
     public virtual Task<string> GetLastPrepLogAsync() => Task.FromResult(LastPrepLog);
+    public WorkerBuildInfoDto? WorkerBuildInfo;
+    public virtual Task<WorkerBuildInfoDto?> GetWorkerBuildInfoAsync() => Task.FromResult(WorkerBuildInfo);
     public virtual Task RefineTaskAsync(string taskId) => Task.CompletedTask;
 
     public virtual Task<OnlineInboxStateDto?> GetOnlineInboxStateAsync() => Task.FromResult<OnlineInboxStateDto?>(null);