Function build_checkpoint_save_tool 

pub fn build_checkpoint_save_tool(
    repo: Arc<Mutex<ShadowRepo>>,
    session_id: String,
    touched_files: Option<Arc<Mutex<TouchedFiles>>>,
    writer: Arc<Mutex<CheckpointLogWriter>>,
    turn_index: Arc<AtomicUsize>,
    log_path: PathBuf,
) -> CheckpointSave

Build a CheckpointSave tool wired to the given shared state.