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.

path_rotate.go

@@ -0,0 +1,54 @@
+package gpg
+
+import (
+	"context"
+	"time"
+
+	"github.com/hashicorp/vault/sdk/framework"
+	"github.com/hashicorp/vault/sdk/logical"
+)
+
+func pathRotate(b *gpgBackend) *framework.Path {
+	return &framework.Path{
+		Pattern: "keys/" + framework.GenericNameRegex("name") + "/rotate",
+		DisplayAttrs: &framework.DisplayAttributes{
+			OperationPrefix: "gpg",
+			OperationSuffix: "key-rotate",
+		},
+		Fields: map[string]*framework.FieldSchema{
+			"name": {
+				Type:        framework.TypeString,
+				Description: "Name of the key.",
+				Required:    true,
+			},
+		},
+		Operations: map[logical.Operation]framework.OperationHandler{
+			logical.UpdateOperation: &framework.PathOperation{Callback: b.pathRotateWrite},
+		},
+		HelpSynopsis:    "Add a new version to a key.",
+		HelpDescription: "Generate a fresh OpenPGP entity as the new latest version. Older versions are retained for decryption and verification.",
+	}
+}
+
+func (b *gpgBackend) pathRotateWrite(ctx context.Context, req *logical.Request, data *framework.FieldData) (*logical.Response, error) {
+	name := data.Get("name").(string)
+
+	lock := b.lockForKey(name)
+	lock.Lock()
+	defer lock.Unlock()
+
+	key, err := b.getKey(ctx, req.Storage, name)
+	if err != nil {
+		return nil, err
+	}
+	if key == nil {
+		return logical.ErrorResponse("key %q not found", name), logical.ErrInvalidRequest
+	}
+	if err := key.rotate(time.Now()); err != nil {
+		return logical.ErrorResponse(err.Error()), logical.ErrInvalidRequest
+	}
+	if err := b.putKey(ctx, req.Storage, key); err != nil {
+		return nil, err
+	}
+	return b.keyResponse(key)
+}