Function prompt_init_install_decision 

pub fn prompt_init_install_decision(
    _cli: &Cli,
    root: &Path,
    args: &InitArgs,
    _json: bool,
) -> Result<Vec<String>>

Pre-write phase of init harness selection: resolve any explicit --install-harnesses request WITHOUT writing anything. Returns the harnesses to install once writes are safe.

--quickstart calls this before any filesystem mutation so scope errors fail before writes. The install itself is deferred to perform_init_install after the writes land. Only the directory root is needed, so it works before the repository exists on disk.