Register + mount the GPG secrets engine at gpg/ (#87)

Complete the deploy of the [vault-plugin-secrets-gpg](https://git.unkin.net/unkin/vault-plugin-secrets-gpg) engine. Puppet ([puppet-prod #480](unkin/puppet-prod#480)) 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.

- Add a `gpg_secret_backend` module using the standard `hashicorp/vault` provider (already required at 5.6.0): `vault_plugin` (catalog register with a pinned sha256) + `vault_mount` (enable at the mount path).
- Wire it through `vault_cluster` (new `gpg_secret_backend` variable + module block) and the config discovery (`config.hcl` group + syd1 terragrunt input), mirroring `litellm_secret_backend`.
- Add `config/gpg_secret_backend/gpg.yaml` mounting at `gpg/` and pinning the released v0.1.0 binary sha256 (`0e92d740…a7b20`, extracted from the published RPM). Puppet installs the RPM floating, so this sha must be bumped in lockstep on any plugin upgrade or OpenBao rejects the binary.

Validated locally with `tofu validate` + `tofu fmt`. Granting non-root access to `gpg/*` (auth roles + policies) is a follow-up scoped to whoever consumes the engine (e.g. passv from CI).

Reviewed-on: #87
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

config/config.hcl

@@ -205,5 +205,18 @@ locals {
       })
       if startswith(file_path, "plugins/")
     }
+    gpg_secret_backend = {
+      for file_path, content in local.all_configs :
+      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/")
+    }
   }
 }