The registry path is lowercase, so the capitalized owner made skopeo inspect
fail and every cosign .sig tag was deleted. Lowercase the owner and, if no
digest resolves, keep all signature tags instead of deleting them.
Add prune-registry.sh: keeps the moving tag and newest versioned tag per
distro plus their cosign signature tags, deletes the rest via the Gitea
packages API. Wire it into the prune job (uses REGISTRY_TOKEN).