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.

src/ClaudeDo.Worker/ClaudeDo.Worker.csproj

@@ -35,4 +35,18 @@
     <InternalsVisibleTo Include="ClaudeDo.Worker.Tests" />
   </ItemGroup>
 
+  <!-- Stamps this worker's exact commit into AssemblyInformationalVersion as "+{sha}" so a
+       running instance can report which commit it was built from (see WorkerHub.GetWorkerBuildInfo).
+       Independent of MinVer: MinVerVersionOverride (release builds) replaces the base version but
+       doesn't touch SourceRevisionId, and the SDK appends it to InformationalVersion regardless. -->
+  <Target Name="SetBuildRevisionFromGit" BeforeTargets="GetAssemblyVersion" Condition="'$(SourceRevisionId)' == ''">
+    <Exec Command="git rev-parse HEAD"
+          WorkingDirectory="$(MSBuildProjectDirectory)"
+          ConsoleToMSBuild="true"
+          StandardOutputImportance="low"
+          ContinueOnError="WarnAndContinue">
+      <Output TaskParameter="ConsoleOutput" PropertyName="SourceRevisionId" />
+    </Exec>
+  </Target>
+
 </Project>

src/ClaudeDo.Worker/Hub/WorkerHub.cs

@@ -29,6 +29,10 @@ namespace ClaudeDo.Worker.Hub;
 
 public record ActiveTaskDto(string Slot, string TaskId, DateTime StartedAt);
 
+/// <summary>Git SHA the running worker was built from. Null when the build wasn't stamped
+/// (e.g. a local dev build without the SourceRevisionId target).</summary>
+public record WorkerBuildInfoDto(string? BuildSha);
+
 public record AppSettingsDto(
     string DefaultClaudeInstructions,
     string DefaultModel,
@@ -159,6 +163,20 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
     private static readonly string Version =
         Assembly.GetExecutingAssembly().GetName().Version?.ToString(3) ?? "0.0.0";
 
+    // SourceRevisionId (set by ClaudeDo.Worker.csproj's git-rev-parse build target) is appended
+    // by the SDK to InformationalVersion as "+{sha}" -- take the segment after the LAST '+' so
+    // this stays correct even when MinVer's own pre-release metadata already contains a '+'.
+    private static readonly string? BuildSha = ParseBuildSha(
+        Assembly.GetExecutingAssembly().GetCustomAttribute<AssemblyInformationalVersionAttribute>()?.InformationalVersion);
+
+    internal static string? ParseBuildSha(string? informationalVersion)
+    {
+        if (string.IsNullOrEmpty(informationalVersion)) return null;
+        var plusIndex = informationalVersion.LastIndexOf('+');
+        if (plusIndex < 0 || plusIndex == informationalVersion.Length - 1) return null;
+        return informationalVersion[(plusIndex + 1)..];
+    }
+
     private readonly QueueService _queue;
     private readonly IQueueWaker _waker;
     private readonly AgentFileService _agentService;
@@ -325,6 +343,11 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
 
     public string Ping() => $"pong v{Version}";
 
+    /// <summary>Lets the UI detect a worker that's still running an older build than the code it's
+    /// comparing against (e.g. a just-merged main) — a separate diagnostic rather than extending
+    /// Ping's "pong vX.Y.Z" text, since that format isn't ours to break for any existing caller.</summary>
+    public WorkerBuildInfoDto GetWorkerBuildInfo() => new(BuildSha);
+
     public IReadOnlyList<ActiveTaskDto> GetActive()
     {
         return _queue.GetActive()