Grant vault deployer access to import + manage the gpg engine (#88)

## Why
Applying the gpg mount (#87) needs two grants the deployer (`tf_vault` approle / `woodpecker_terraform_vault` k8s role) doesn't have. terraform-vault **registers the plugin itself** (`vault_plugin` → `sys/plugins/catalog`, a sudo-protected path) and **manages keys** via the gpgvaultsecret provider (`gpg/keys/*`). The deployer already has `sys/mounts/*` but neither of these, so apply would 403 on the plugin registration and on `gpg/keys` writes — the same failure mode as #84.

## Changes
- Add `policies/gpg/admin.yaml` granting:
  - `create/read/update/delete/sudo` on `sys/plugins/catalog/secret/vault-plugin-secrets-gpg` — to **import** (register/deregister) the plugin.
  - full management of `gpg/keys/*` (+ `gpg/keys` list) — to **manage keys**.
  - assigned to `tf_vault` (approle) + `woodpecker_terraform_vault` (k8s/au/syd1), mirroring `policies/litellm/admin.yaml` (#84).

Should merge/apply **before** #87 so the deployer can register the plugin and create the `pass` key.

Reviewed-on: #88
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

policies/gpg/admin.yaml

@@ -0,0 +1,35 @@
+# Allow the vault deployer to import the gpg plugin and manage its OpenPGP keys.
+#
+# terraform-vault registers the plugin itself (vault_plugin -> sys/plugins/catalog,
+# a sudo-protected path) and manages keys via the gpgvaultsecret provider, so the
+# deployer needs catalog access on top of the mount access it already has
+# (sys/mounts/*). Without this, apply 403s on the plugin registration and on
+# gpg/keys writes.
+---
+rules:
+  # Import / register (and deregister) the gpg plugin in the catalog.
+  - path: "sys/plugins/catalog/secret/vault-plugin-secrets-gpg"
+    capabilities:
+      - create
+      - read
+      - update
+      - delete
+      - sudo
+  # Manage keys (create/rotate/config/delete) in the gpg mount.
+  - path: "gpg/keys/*"
+    capabilities:
+      - create
+      - read
+      - update
+      - delete
+      - list
+  - path: "gpg/keys"
+    capabilities:
+      - read
+      - list
+
+auth:
+  approle:
+    - tf_vault
+  k8s/au/syd1:
+    - woodpecker_terraform_vault