Craigslist ads, Facebook posts and The Negev’s own website tout 219 Sixth St. as the epitome of modern communal living in San Francisco — a like-minded group of people dedicated to entrepreneurship, engineering, weekly tech talks, family dinners and partying. While that might be true, there is a different side to life behind the bright-red metal gate of The Negev Sixth. Nearly all tenants in the single-room-occupancy building — mostly in their 20s and newcomers…