Add mcrctl purge command for tag retention

Client-side purge that keeps the last N tags per repository (excluding
latest) and deletes older manifests. Uses existing MCR APIs — no new
server RPCs needed.

Server-side: added updated_at to TagInfo struct and GetRepositoryDetail
query so tags can be sorted by recency.

Usage: mcrctl purge --keep 3 --dry-run --gc

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/grpcserver/registry.go

@@ -78,8 +78,9 @@ func (s *registryService) GetRepository(_ context.Context, req *pb.GetRepository
 	}
 	for _, t := range detail.Tags {
 		resp.Tags = append(resp.Tags, &pb.TagInfo{
-			Name:   t.Name,
-			Digest: t.Digest,
+			Name:      t.Name,
+			Digest:    t.Digest,
+			UpdatedAt: t.UpdatedAt,
 		})
 	}
 	for _, m := range detail.Manifests {