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.Data/Git/GitService.cs

@@ -26,6 +26,22 @@ public sealed class GitService
         return stdout.Trim();
     }
 
+    /// <summary>
+    /// True if <paramref name="ancestorSha"/> is an ancestor of (or equal to) <paramref name="descendantSha"/>,
+    /// via `git merge-base --is-ancestor`. Null means the answer can't be determined (e.g. the commit is
+    /// unknown in this repo) — callers must treat that as "unknown", never as "not an ancestor".
+    /// </summary>
+    public async Task<bool?> IsAncestorAsync(string repoDir, string ancestorSha, string descendantSha, CancellationToken ct = default)
+    {
+        var (exitCode, _, _) = await RunGitAsync(repoDir, ["merge-base", "--is-ancestor", ancestorSha, descendantSha], ct);
+        return exitCode switch
+        {
+            0 => true,
+            1 => false,
+            _ => null,
+        };
+    }
+
     public async Task WorktreeAddAsync(string repoDir, string branchName, string worktreePath, string baseCommit, CancellationToken ct = default)
     {
         await WorktreeAddGate.WaitAsync(ct);