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"
 

modules/vault_cluster/modules/gpg_secret_backend/main.tf

@@ -0,0 +1,20 @@
+# Registers the GPG/OpenPGP secrets plugin in the catalog and mounts it. The
+# plugin binary is installed on the OpenBao nodes by Puppet
+# (openbao-plugin-secrets-gpg RPM -> /opt/openbao-plugins/vault-plugin-secrets-gpg);
+# `command` is that filename, resolved relative to the server's plugin_directory.
+#
+# The sha256 is pinned here to the released binary. Puppet installs the RPM
+# floating, so on a plugin upgrade this sha256 must be bumped in lockstep or
+# OpenBao will refuse to launch the plugin.
+resource "vault_plugin" "this" {
+  type    = "secret"
+  name    = var.plugin
+  command = var.command
+  sha256  = var.sha256
+}
+
+resource "vault_mount" "this" {
+  path        = var.path
+  type        = vault_plugin.this.name
+  description = var.description
+}

modules/vault_cluster/modules/gpg_secret_backend/terraform.tf

@@ -0,0 +1,9 @@
+terraform {
+  required_version = ">= 1.10"
+  required_providers {
+    vault = {
+      source  = "hashicorp/vault"
+      version = "5.6.0"
+    }
+  }
+}

modules/vault_cluster/modules/gpg_secret_backend/variables.tf

@@ -0,0 +1,27 @@
+variable "path" {
+  description = "Mount path of the GPG secrets engine (e.g. \"gpg\")"
+  type        = string
+}
+
+variable "plugin" {
+  description = "Name to register the plugin under in the catalog (also the mount type)"
+  type        = string
+  default     = "vault-plugin-secrets-gpg"
+}
+
+variable "command" {
+  description = "Plugin binary filename, relative to the server plugin_directory"
+  type        = string
+  default     = "vault-plugin-secrets-gpg"
+}
+
+variable "sha256" {
+  description = "SHA-256 of the installed plugin binary; must match the on-disk binary or OpenBao rejects it"
+  type        = string
+}
+
+variable "description" {
+  description = "Human-friendly description of the mount"
+  type        = string
+  default     = null
+}

modules/vault_cluster/variables.tf

@@ -315,6 +315,17 @@ variable "litellm_secret_backend_role" {
   default = {}
 }
 
+variable "gpg_secret_backend" {
+  description = "Map of GPG/OpenPGP secret engines to register + mount (path => plugin sha256 + config)"
+  type = map(object({
+    plugin      = optional(string, "vault-plugin-secrets-gpg")
+    command     = optional(string, "vault-plugin-secrets-gpg")
+    sha256      = string
+    description = optional(string)
+  }))
+  default = {}
+}
+
 variable "policy_auth_map" {
   description = "Map of auth mounts -> auth roles -> policy names"
   type        = map(map(list(string)))