Scaffold ghp secrets engine modelled on vault-plugin-secrets-gitea

Mints ephemeral, scoped ghp access tokens via ghp's admin token API
(POST /api/tokens), bound to a Vault lease and revoked on lease
expiry (DELETE /api/tokens/{id}).

- config: base_url + write-only admin_token (ghpsvc_ service token),
  TLS settings; verifies the token is a ghp admin on write. No
  rotate-root: the service token is static and operator-managed.
- roles: token_type (agent/proxy), installation_id, app_record_id,
  repositories, scopes (permission:level), session_prefix, ttl/max_ttl.
- creds: mint a lease-bound token; ghp-side duration bounded by the
  lease ceiling as defence in depth.
- secret ghp_token: idempotent revoke + lease renew.
- Unit tests (config/role/creds/client/scopes/revocation), mock-ghp
  e2e on Vault + OpenBao, Woodpecker pre-commit/build/test/release,
  Makefile patch/minor/major, nfpm RPM packaging.

packaging/scripts/preinstall.sh.tmpl

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+# Ensure the plugin directory exists before the binary is laid down.
+# Rendered per flavour via envsubst (see scripts/build-rpm.sh).
+mkdir -p ${PACKAGE_PLUGIN_DIR}