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>

proto/mcr/v1/registry.proto

@@ -25,6 +25,7 @@ message RepositoryMetadata {
 message TagInfo {
   string name = 1;
   string digest = 2;
+  string updated_at = 3;
 }
 
 message ManifestInfo {