Function append_prompt_arg 

pub(super) fn append_prompt_arg(
    args: &mut Vec<String>,
    flag: &str,
    prompt: &str,
) -> Result<(), String>

For a provider that takes its prompt as an argument rather than on stdin (Antigravity: agy -p <prompt>), append <flag> <prompt> last, after any resume flag. A prompt too long for the command line is refused, never truncated: a cut-off prompt would run as something the user didn’t send.