RouterBase PHP Client 0.1.0

Initial PHP client release for RouterBase.\n\nIncludes:\n- Composer package metadata for routerbase/client.\n- Dependency-free PHP client for chat completions and model listing.\n- CLI entry point: routerbase-chat.\n- Plain PHP tests with fake transport.\n- GitHub Actions CI across PHP 8.1, 8.2, and 8.3.\n\nDocs: https://docs.routerbase.com/