THE city of New York owns thousands of slivers of unused land, and about a year ago, a group of Brooklyn gardeners had an idea: identify all the vacant lots in the borough, then help neighborhood residents take them over. They built an online map, then a mobile app, with information about the plots, including the names and phone numbers of the agencies that owned them. They called themselves596 Acres, after the total area of unused public land in Brooklyn, according to city data./.../