Add app-token JWT secrets engine

Implement a generic Vault/OpenBao secrets engine that issues short-lived
signed JWTs for self-made services, replacing per-app static bearer
Secrets. Per-app roles set the audience, TTLs, subject allowlist and
custom claims; creds/<role> mints an EdDSA (or RS256) token. Apps
validate offline against the unauthenticated JWKS + OIDC-metadata paths,
so there is no Vault round-trip per request. Signing keys are seal-wrapped
in the barrier and rotate with a configurable JWKS grace window.

Mirrors the other vault-plugin-secrets-* engines: cmd ServeMultiplex,
Makefile with patch|minor|major tags, .woodpecker CI (k8s resources + SA),
dual-flavour nfpm RPM to artifactapi rpm-internal. Tests (-race) cover
issuance+JWKS validation for both algorithms, rotation grace/trim, role
isolation, subject allowlist and the unauthenticated/seal-wrap wiring.

Claude-Session: https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT

cmd/vault-plugin-secrets-apptoken/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"
+
+	apptoken "git.unkin.net/unkin/vault-plugin-secrets-apptoken"
+)
+
+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: apptoken.Factory,
+		TLSProviderFunc:    tlsProviderFunc,
+	})
+	if err != nil {
+		logger := hclog.New(&hclog.LoggerOptions{})
+		logger.Error("plugin shutting down", "error", err)
+		os.Exit(1)
+	}
+}