Initial vault-plugin-secrets-rancher scaffold

Vault/OpenBao secrets engine managing Rancher API tokens via the public
tokens.ext.cattle.io API.

- config: Rancher connection (URL + TLS)
- service-accounts/<name>: seeded root tokens, auto-rotated before Rancher's
  TTL cap via a PeriodicFunc (default 45d rotation, 90d token TTL); the current
  token mints its own replacement. Manual /rotate endpoint too.
- roles/<name>: mint policy referencing a service account; cluster_name + TTL
  scoping (Rancher tokens inherit the seeding user's RBAC).
- creds/<role>: dynamic, lease-bound tokens deleted from Rancher on revoke.

Ports the bind-tsig Woodpecker RPM release, nfpm packaging, and a mock-Rancher
e2e (Vault + OpenBao). Unit tests cover the full lifecycle.

packaging/nfpm.yaml

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

packaging/scripts/preinstall.sh.tmpl

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+#!/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}