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.

cmd/vault-plugin-secrets-gpg/main.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"os"
+
+	hclog "github.com/hashicorp/go-hclog"
+	"github.com/hashicorp/vault/api"
+	"github.com/hashicorp/vault/sdk/plugin"
+
+	gpg "git.unkin.net/unkin/vault-plugin-secrets-gpg"
+)
+
+func main() {
+	apiClientMeta := &api.PluginAPIClientMeta{}
+	flags := apiClientMeta.FlagSet()
+	if err := flags.Parse(os.Args[1:]); err != nil {
+		logger := hclog.New(&hclog.LoggerOptions{})
+		logger.Error("failed to parse flags", "error", err)
+		os.Exit(1)
+	}
+
+	tlsConfig := apiClientMeta.GetTLSConfig()
+	tlsProviderFunc := api.VaultPluginTLSProvider(tlsConfig)
+
+	err := plugin.ServeMultiplex(&plugin.ServeOpts{
+		BackendFactoryFunc: gpg.Factory,
+		TLSProviderFunc:    tlsProviderFunc,
+	})
+	if err != nil {
+		logger := hclog.New(&hclog.LoggerOptions{})
+		logger.Error("plugin shutting down", "error", err)
+		os.Exit(1)
+	}
+}