LAS VEGAS — A Nevada lawmaker, released following a five-day mental evaluation and allegations that he threatened a state Democratic Assembly leader, vowed Friday to serve his elected office when the state Legislature convenes on Monday.Legislative leaders, meanwhile, took the unprecedented step of forming an investigative panel to consider whether Assemblyman Steven Brooks is fit to take his seat. The select committee will recommend to the Assembly whether any action should be taken.Brooks spoke nearly non-stop for about three minutes then hung up the telephone when reached at home by The Associated Press.The Democratic assemblyman from North Las Vegas identified …