Function spawn_config_dir 

fn spawn_config_dir(
    provider: &str,
    env: &HashMap<String, String>,
) -> Option<String>

The config dir the spawn gives its provider: the provider’s own auth_config_dir_env_var (Gemini’s is GEMINI_CLI_HOME, not the GEMINI_CONFIG_DIR a hand-kept list here once named), else the first known provider’s variable the env carries.