Add GPG/OpenPGP secrets engine

Provide a transit-style Vault/OpenBao secrets engine whose key material is
OpenPGP, so services can sign/verify/encrypt/decrypt with GPG keys that never
leave the barrier — and so tools like pass can encrypt to an exported public
key while delegating decryption back to Vault.

- Add versioned key management (keys/<name> CRUD+list, config, rotate, import)
  with private material seal-wrapped under key/ and per-key locking.
- Add sign/verify (detached OpenPGP) and encrypt/decrypt paths; decrypt
  auto-detects armored vs raw-binary ciphertext (what pass/gpg write).
- Add export/<public-key|private-key>/<name>; public always exportable,
  private only when the key is marked exportable.
- Use ProtonMail go-crypto for OpenPGP; support rsa-2048/3072/4096 and ed25519.
- Clone the sibling plugin's build/packaging/CI: dual-target RPMs
  (vault + openbao plugin dirs), Woodpecker PR/release pipelines, and a
  Vault+OpenBao e2e harness. Unit tests include real gpg and pass interop.

scripts/build-rpm.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+#
+# Package the (already built) plugin binary into RPMs with nfpm. Builds one RPM
+# per target server: Vault (/opt/vault-plugins) and OpenBao (/opt/openbao-plugins).
+# Usage: scripts/build-rpm.sh [version]   (version defaults to $CI_COMMIT_TAG)
+#
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "${ROOT_DIR}"
+
+VERSION="${1:-${CI_COMMIT_TAG:-0.0.0-dev}}"
+VERSION="${VERSION#v}"
+BINARY="vault-plugin-secrets-gpg"
+DIST="dist"
+
+if [ ! -f "${DIST}/${BINARY}" ]; then
+  echo "ERROR: ${DIST}/${BINARY} not found; run 'make build' first" >&2
+  exit 1
+fi
+
+export PACKAGE_VERSION="${VERSION}"
+export PACKAGE_RELEASE="1"
+export PACKAGE_ARCH="amd64"
+export PACKAGE_PLATFORM="linux"
+export PACKAGE_DESCRIPTION="Vault/OpenBao secrets engine for GPG/OpenPGP keys (sign/verify/encrypt/decrypt)"
+export PACKAGE_MAINTAINER="Ben Vincent <ben@unkin.net>"
+export PACKAGE_HOMEPAGE="https://git.unkin.net/unkin/vault-plugin-secrets-gpg"
+export PACKAGE_LICENSE="MIT"
+
+build_flavor() {
+  export PACKAGE_NAME="$1"
+  export PACKAGE_PLUGIN_DIR="$2"
+  export PACKAGE_PREINSTALL="${DIST}/preinstall-${PACKAGE_NAME}.sh"
+  envsubst '${PACKAGE_PLUGIN_DIR}' < packaging/scripts/preinstall.sh.tmpl > "${PACKAGE_PREINSTALL}"
+  envsubst < packaging/nfpm.yaml > "${DIST}/nfpm-${PACKAGE_NAME}.yaml"
+  nfpm pkg --config "${DIST}/nfpm-${PACKAGE_NAME}.yaml" --target "${DIST}" --packager rpm
+}
+
+build_flavor "vault-plugin-secrets-gpg"   "/opt/vault-plugins"
+build_flavor "openbao-plugin-secrets-gpg" "/opt/openbao-plugins"
+
+echo "Built:"
+ls -1 "${DIST}"/*.rpm