Function create_session 

pub fn create_session(
    proj_name: &str,
    wt_slug: &str,
    wt_path: &PathBuf,
    session_name: Option<String>,
    command: Option<String>,
) -> Result<(String, String)>

Create a named tmux session at wt_path and optionally send an initial command. Returns (tmux_name, display_name). Tmux name is prefixed with {proj_name}-{wt_slug}-; display_name is the user-visible part (what the user typed).