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.

Init calls this before installing harnesses so scope errors fail before integration files are written. The install itself is deferred to perform_init_install after the repository is created. Only the directory root is needed, so it works before the repository exists on disk.