Heartbroken friends and coworkers of Caitlin Clavette — a beloved Milton elementary school art teacher who was tragically killed yesterday when a 200-pound manhole cover came crashing through her windshield — are remembering her as a gifted triathlete with an upbeat attitude that was contagious.“She was the kind of person who would show up to every team event with a big smile on her face,” said Anne Traer, president of the Boston-based Wheelworks Multisport Triathlon Team.