Function save_session_full 

pub fn save_session_full(
    session_id: &str,
    messages: &[Message],
    cwd: &str,
    model: &str,
    turn_count: usize,
    total_cost_usd: f64,
    total_input_tokens: u64,
    total_output_tokens: u64,
    plan_mode: bool,
) -> Result<PathBuf, String>

Save the full session state to disk (including cost and token tracking).