Function detect_cli_version 

pub fn detect_cli_version(bin: &str, cache_file: &str, fallback: &str) -> String

Detects an installed CLI’s version by running <bin> --version, caching the result under ~/.vct/<cache_file> for the day so it is not re-run on every launch. Falls back to fallback when the CLI is absent or unreadable, so the User-Agent it feeds is always a plausible client version — and returns that same fallback outright until enable_cli_version_detection is called.