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.

environments/au/syd1/terragrunt.hcl

@@ -71,6 +71,7 @@ inputs = {
   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
+  gpg_key                        = local.config.gpg_key
 
   # Pass policy maps to vault_cluster module
   policy_auth_map  = local.policies.policy_auth_map