This fall, three Somerville non-profits and the Somerville Housing Authority are celebrating an unusual harvest at the Mystic Housing Developments. Normally, of course, there’s a harvest of vegetables and flowers from the immigrant-led Mystic Community Garden. But this year, it’s something more – a harvest from seeds sewn through new programming for Mystic youth, and a harvest reaped from new collaboration among Somerville non-profits this spring and summer.