An MS-13 gang member has admitted he pretended to be a girl on Facebook in 2015 to lure a 15-year-old boy to Constitution Beach in East Boston, where the victim was stabbed to death, prosecutors said.Carlos “Chuchito” “Criminal” Melara, a 21-year-old Salvadoran national, pleaded guilty Friday in federal court in Boston to conspiracy to conduct enterprise affairs through a pattern of racketeering activity, according to U.S.