Scaffold the bind-tsig secrets engine

A Vault/OpenBao secrets engine that manages BIND TSIG keys via the
bind-operator companion API (Vault -> HTTP API -> BindTSIGKey CRs).

- backend + cmd entry point (plugin.ServeMultiplex), modelled on
  vault-plugin-secrets-litellm
- config path: companion API url/token/tls + defaults
- static-roles/static-creds: stable named key with managed rotation
- roles/creds: dynamic, lease-bound keys (revoke deletes the CR)
- tsig_key secret type with revoke/renew
- HTTP client for the companion API contract (/v1/keys CRUD + rotate)
- Makefile, Woodpecker CI (pre-commit/build/test + tag release RPMs),
  nfpm packaging (vault + openbao flavours)
- e2e: mock companion API + Vault + OpenBao in docker-compose, full
  lifecycle per engine; unit tests for the dynamic + static flows

packaging/nfpm.yaml

@@ -0,0 +1,35 @@
+---
+# nfpm config for the vault-plugin-secrets-bind-tsig 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-bind-tsig
+    dst: ${PACKAGE_PLUGIN_DIR}/vault-plugin-secrets-bind-tsig
+    file_info:
+      mode: 0755
+      owner: root
+      group: root
+
+scripts:
+  preinstall: ${PACKAGE_PREINSTALL}

packaging/scripts/preinstall.sh.tmpl

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+# Ensure the plugin directory exists before the binary is laid down.
+# Rendered per flavour via envsubst (see scripts/build-rpm.sh).
+mkdir -p ${PACKAGE_PLUGIN_DIR}