Initial vault-plugin-secrets-netbox

Vault/OpenBao secrets engine that mints NetBox API tokens via
/api/users/tokens/. A single seeded admin token (config) mints short-lived,
per-user tokens (roles -> creds) whose NetBox expiry is aligned to the Vault
lease; revoke deletes the token, renew extends its expiry. config/rotate
reissues the seeded admin token. Handles NetBox 4.6 v2 tokens (Bearer
nbt_<key>.<secret>) and legacy v1. Unit tests against an httptest NetBox mock;
dual Vault/OpenBao RPMs via nfpm; tag-driven release to artifactapi.

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

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}