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.

modules/vault_cluster/main.tf

@@ -334,6 +334,18 @@ module "litellm_secret_backend_role" {
   depends_on = [module.litellm_secret_backend]
 }
 
+module "gpg_secret_backend" {
+  source = "./modules/gpg_secret_backend"
+
+  for_each = var.gpg_secret_backend
+
+  path        = each.key
+  plugin      = each.value.plugin
+  command     = each.value.command
+  sha256      = each.value.sha256
+  description = each.value.description
+}
+
 module "vault_policy" {
   source = "./modules/vault_policy"