Deploy the GPG/OpenPGP secrets engine to the OpenBao (vault-role) cluster by
installing its plugin RPM into /opt/openbao-plugins, mirroring the existing
openbao-plugin-secrets-litellm deployment. The RPM ships from artifactapi
rpm-internal (built on the vault-plugin-secrets-gpg v0.1.0 tag).
Registering/enabling the backend is a separate terraform-vault change.