Register and mount the GPG secrets engine at gpg/

Puppet installs the openbao-plugin-secrets-gpg RPM onto the OpenBao nodes; this
registers that binary in the plugin catalog and enables the secrets engine so
gpg/ is actually usable (encrypt/decrypt/sign/verify).

- Add a gpg_secret_backend module using the standard vault provider:
  vault_plugin (catalog register with a pinned sha256) + vault_mount.
- Wire it through vault_cluster (variable + module) and the config discovery
  (config.hcl + syd1 terragrunt input), mirroring litellm_secret_backend.
- Add config/gpg_secret_backend/gpg.yaml pinning the released v0.1.0 binary
  sha256 (0e92d740...a7b20). Puppet installs the RPM floating, so this sha
  must be bumped in lockstep on any plugin upgrade.

Granting non-root access to gpg/* (auth roles + policies) is a follow-up,
scoped to whoever consumes the engine.

environments/au/syd1/terragrunt.hcl

@@ -70,6 +70,7 @@ inputs = {
   pki_mount_only                 = local.config.pki_mount_only
   litellm_secret_backend         = local.config.litellm_secret_backend
   litellm_secret_backend_role    = local.config.litellm_secret_backend_role
+  gpg_secret_backend             = local.config.gpg_secret_backend
 
   # Pass policy maps to vault_cluster module
   policy_auth_map  = local.policies.policy_auth_map