Add a gpg_key module and a 'pass' key for password-store

Manage OpenPGP keys in the gpg engine from Terraform using the new
gpgvaultsecret provider, and create the first key ('pass') for passv.

- Add a gpg_key module wrapping the gpg_key resource; wire it through
  vault_cluster (variable + module, depends_on the mount) and config discovery
  (config.hcl group keyed <backend>/<name>, syd1 terragrunt input), mirroring
  the *_secret_backend_role modules.
- Register the gpg provider in root.hcl (provider block + required_providers,
  v0.1.0 from the terraform-unkin registry), alongside litellm.
- Add config/gpg_key/gpg/pass.yaml: an rsa-4096 key 'pass' in the gpg mount for
  password-store. Private key stays in Vault; clients import the public key and
  delegate decryption to gpg/decrypt/pass.

modules/vault_cluster/main.tf

@@ -346,6 +346,22 @@ module "gpg_secret_backend" {
   description = each.value.description
 }
 
+module "gpg_key" {
+  source = "./modules/gpg_key"
+
+  for_each = var.gpg_key
+
+  backend                = each.value.backend
+  name                   = each.value.name
+  algorithm              = each.value.algorithm
+  identity               = each.value.identity
+  exportable             = each.value.exportable
+  deletion_allowed       = each.value.deletion_allowed
+  min_decryption_version = each.value.min_decryption_version
+
+  depends_on = [module.gpg_secret_backend]
+}
+
 module "vault_policy" {
   source = "./modules/vault_policy"