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.

packaging/nfpm.yaml

@@ -0,0 +1,35 @@
+---
+# nfpm config for the vault-plugin-secrets-gpg RPM. Rendered through
+# envsubst (see scripts/build-rpm.sh) then fed to `nfpm pkg`. Built once per
+# target server (Vault, OpenBao); PACKAGE_NAME and PACKAGE_PLUGIN_DIR vary.
+
+name: ${PACKAGE_NAME}
+version: ${PACKAGE_VERSION}
+release: ${PACKAGE_RELEASE}
+arch: ${PACKAGE_ARCH}
+platform: ${PACKAGE_PLATFORM}
+section: default
+priority: extra
+description: "${PACKAGE_DESCRIPTION}"
+
+maintainer: ${PACKAGE_MAINTAINER}
+homepage: ${PACKAGE_HOMEPAGE}
+license: ${PACKAGE_LICENSE}
+
+disable_globbing: false
+
+replaces:
+  - ${PACKAGE_NAME}
+provides:
+  - ${PACKAGE_NAME}
+
+contents:
+  - src: dist/vault-plugin-secrets-gpg
+    dst: ${PACKAGE_PLUGIN_DIR}/vault-plugin-secrets-gpg
+    file_info:
+      mode: 0755
+      owner: root
+      group: root
+
+scripts:
+  preinstall: ${PACKAGE_PREINSTALL}