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/root.hcl

@@ -17,6 +17,12 @@ provider "litellm" {
   address = local.vault_addr
 }
 
+# The gpg secrets engine's keys are managed through its own provider (same Vault
+# server; token falls back to VAULT_TOKEN).
+provider "gpg" {
+  address = local.vault_addr
+}
+
 terraform {
   backend "consul" {
     address = "https://consul.service.consul"
@@ -39,6 +45,10 @@ terraform {
       source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/litellmvaultsecret"
       version = "0.1.0"
     }
+    gpg = {
+      source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/gpgvaultsecret"
+      version = "0.1.0"
+    }
   }
 }
 EOF