Function print_error_envelope 

pub fn print_error_envelope(
    exit_code: i32,
    error_code: &str,
    message: &str,
    remote_exit_code: Option<i32>,
    error_class: ErrorClass,
    retryable: bool,
    suggestion: Option<&str>,
) -> Result<()>

Emits a JSON error envelope on stderr (GAP-SSH-IO-003 / G-RETRY / G-ERR-08).