Restricted free agent Marcus Smart has agreed to a four-year, $52 million contract extension with the Boston Celtics, league sources told ESPN's Adrian Wojnarowski. Smart, the No. 6 pick in the 2014 draft, found a quiet market in restricted free agency and, after his camp grumbled about Boston's lack of attention early in the summer, the two sides eventually hammered out a deal. The deal is a victory for Smart's camp, which turned down a similar extension offer before the start of the season, according to sources. With Smart receiving no formal offer sheets this month at the start of restricted free agency, the Celtics could have simply waited for Smart and his agent to settle for a $6.1 million qualifying offer by Oct. 1. Instead, the Celtics gain some security at point guard, one summer before All-Star  Kyrie Irving is scheduled to reach unrestricted free agency. ( Terry Rozier would be a restricted free agent if no extension is reached). Boston...