Boston Celtics forward Jaylen Brown rocked the best Halloween costume we’ve seen in a while. On Monday he dressed up like fitness guru Richard Simmons and helped lead an exercise class for seniors at the Roxbury Tenants of Harvard Community Center in Mission Hill. Brown was joined by former Celtic Tom “Satch’’ Sanders and Celtics team mascot “Lucky’’ the Leprechaun at the event, which was presented by New England Baptist Hospital.