Function make_prompt 

pub fn make_prompt(
    isl_tokens: usize,
    mode: PromptMode,
    prefix_tag: &str,
) -> String

2026-09-26: Build a prompt of roughly isl_tokens tokens. A non-empty prefix_tag opens the prompt as [tag] : a unique tag gives a prompt no earlier request shares, and a constant tag gives the same prompt every time.