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.

config/config.hcl

@@ -203,5 +203,13 @@ locals {
       trimsuffix(basename(file_path), ".yaml") => content
       if startswith(file_path, "gpg_secret_backend/")
     }
+    gpg_key = {
+      for file_path, content in local.all_configs :
+      trimsuffix(replace(file_path, "gpg_key/", ""), ".yaml") => merge(content, {
+        name    = trimsuffix(basename(file_path), ".yaml")
+        backend = dirname(replace(file_path, "gpg_key/", ""))
+      })
+      if startswith(file_path, "gpg_key/")
+    }
   }
 }

config/gpg_key/gpg/pass.yaml

@@ -0,0 +1,7 @@
+# config/gpg_key/gpg/pass.yaml
+# An OpenPGP key in the gpg engine for password-store (passv). The private key
+# stays in Vault; clients import the exported public key to encrypt and delegate
+# decryption to gpg/decrypt/pass. Key name = "pass", backend = "gpg".
+algorithm: rsa-4096
+identity: "pass <pass@unkin.net>"
+exportable: false