Function save_cache 

pub fn save_cache(
    workspace: &WorkspaceState,
    tree_selected: usize,
    flat: &[FlatEntry],
    command_history: &[String],
    active_tab: Option<&str>,
    sync: bool,
) -> Option<String>

Persist session names, expand states, cursor position, active_tab, and command history. Returns an error string if the save fails (caller should surface it in TUI).