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

test/vault/vault.hcl

@@ -0,0 +1,4 @@
+# Combined with `-dev` at runtime; supplies the plugin_directory the dev server
+# would otherwise leave unset, so the plugin binary in ../dist can be registered.
+plugin_directory = "/vault/plugins"
+api_addr         = "http://127.0.0.1:8200"