Family members of a 15-year-old Chelsea boy gunned down in the street more than a year ago sat in a Boston courtroom yesterday waiting for their first glimpse of Jimmy Vasquez’s alleged killer — only to have the young man hide from their sight behind a metal door.Juan Carlos Matos Figueroa, 21, his voice echoing from a prisoner passageway, pleaded “not guilty” to more than a dozen charges, including murder and five counts of armed assault with intent to kill.