Eliminating Kubernetes Image Signature Replication

The Kubernetes image promoter pipeline has been simplified by eliminating the signature replication job that previously copied cosign signatures across 22 regional Google Artifact Registry backends. Instead of replicating every .sig and .att artifact to all regions, the archeio redirector was updated to route all signature manifest requests to a single canonical region (us-central1). This removes thousands of API calls every 2 hours, eliminates a source of transient failures from rate limiting, and deletes over 1,200 lines of replication code. The trade-off is a single-region dependency for cosign verify, which is considered acceptable given Google Cloud's regional availability guarantees. End users see no change in behavior.