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/e2e.sh

@@ -0,0 +1,100 @@
+#!/usr/bin/env bash
+#
+# End-to-end test for vault-plugin-secrets-gpg.
+#
+# Builds the plugin, brings up both Vault and OpenBao, then drives the identical
+# lifecycle against each to prove the same binary works on both:
+#   generate key -> sign/verify -> encrypt/decrypt -> rotate -> decrypt old
+#   ciphertext -> export public key.
+#
+# Select engines with ENGINES (default "vault openbao"), e.g. ENGINES=openbao.
+#
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+COMPOSE_FILE="${ROOT_DIR}/test/docker-compose.yml"
+COMPOSE="docker compose -f ${COMPOSE_FILE}"
+BINARY="vault-plugin-secrets-gpg"
+
+MOUNT="gpg"
+ENGINES="${ENGINES:-vault openbao}"
+
+red()   { printf '\033[31m%s\033[0m\n' "$*"; }
+green() { printf '\033[32m%s\033[0m\n' "$*"; }
+blue()  { printf '\033[34m==> %s\033[0m\n' "$*"; }
+
+cleanup() { blue "Tearing down containers"; ${COMPOSE} down -v >/dev/null 2>&1 || true; }
+trap cleanup EXIT
+fail() { red "FAIL: $*"; exit 1; }
+
+wait_for() {
+  local desc="$1"; shift
+  local i=0
+  until "$@" >/dev/null 2>&1; do
+    i=$((i + 1))
+    [ "$i" -ge "${WAIT_RETRIES:-90}" ] && fail "timed out waiting for ${desc}"
+    sleep 2
+  done
+  green "ready: ${desc}"
+}
+
+b64() { printf '%s' "$1" | base64 | tr -d '\n'; }
+
+run_engine() {
+  local engine="$1" container="$2" cli="$3"
+  blue "[${engine}] exercising the plugin"
+  ex() { ${COMPOSE} exec -T "${container}" "${cli}" "$@"; }
+
+  local sha; sha="$(sha256sum "${ROOT_DIR}/dist/${BINARY}" | awk '{print $1}')"
+  ex plugin register -sha256="${sha}" secret "${BINARY}" >/dev/null || true
+  ex secrets disable "${MOUNT}" >/dev/null 2>&1 || true
+  ex secrets enable -path="${MOUNT}" "${BINARY}" >/dev/null
+  green "[${engine}] plugin registered and mounted"
+
+  # --- generate a key ---
+  local pub
+  pub="$(ex write -field=public_key "${MOUNT}/keys/app" algorithm=rsa-2048 identity='App <app@unkin.net>')"
+  printf '%s' "${pub}" | grep -q 'BEGIN PGP PUBLIC KEY BLOCK' || fail "[${engine}] no armored public key returned"
+  green "[${engine}] key generated; public key exported"
+
+  # --- sign / verify ---
+  local msg sig valid
+  msg="$(b64 'ship it')"
+  sig="$(ex write -field=signature "${MOUNT}/sign/app" input="${msg}")"
+  valid="$(ex write -field=valid "${MOUNT}/verify/app" input="${msg}" signature="${sig}")"
+  [ "${valid}" = "true" ] || fail "[${engine}] signature did not verify"
+  green "[${engine}] sign/verify ok"
+
+  # --- encrypt / decrypt roundtrip ---
+  local ct pt secret="correct horse battery staple"
+  ct="$(ex write -field=ciphertext "${MOUNT}/encrypt/app" plaintext="$(b64 "${secret}")")"
+  pt="$(ex write -field=plaintext "${MOUNT}/decrypt/app" ciphertext="${ct}" | base64 -d)"
+  [ "${pt}" = "${secret}" ] || fail "[${engine}] encrypt/decrypt roundtrip mismatch"
+  green "[${engine}] encrypt/decrypt ok"
+
+  # --- rotate, then decrypt the pre-rotation ciphertext ---
+  local ver
+  ver="$(ex write -field=latest_version "${MOUNT}/keys/app/rotate")"
+  [ "${ver}" = "2" ] || fail "[${engine}] rotate did not bump to version 2"
+  pt="$(ex write -field=plaintext "${MOUNT}/decrypt/app" ciphertext="${ct}" | base64 -d)"
+  [ "${pt}" = "${secret}" ] || fail "[${engine}] old ciphertext failed to decrypt after rotation"
+  green "[${engine}] rotation retains old versions"
+
+  green "[${engine}] PASSED"
+}
+
+blue "Building plugin for linux/amd64"
+OS=linux ARCH=amd64 PLUGIN_DIR="${ROOT_DIR}/dist" make -C "${ROOT_DIR}" build
+
+blue "Starting Docker stack (vault + openbao)"
+${COMPOSE} up -d --build
+
+for engine in ${ENGINES}; do
+  case "${engine}" in
+    vault)   wait_for "vault"   ${COMPOSE} exec -T vault   vault status -address=http://127.0.0.1:8200; run_engine vault vault vault ;;
+    openbao) wait_for "openbao" ${COMPOSE} exec -T openbao bao   status -address=http://127.0.0.1:8200; run_engine openbao openbao bao ;;
+    *) fail "unknown engine: ${engine}" ;;
+  esac
+done
+
+green "ALL END-TO-END CHECKS PASSED (${ENGINES})"