SOUTH SAN FRANCISCO — Residents warned to avoid drinking or cooking with tap water after a water tank security breach received the all-clear late Saturday afternoon. “Everything is clean,” Darin Duncan, Cal Water’s field operations director, said after comprehensive testing showed no contamination of the water source. Residents of South San Francisco’s Sunshine Gardens neighborhood and Colma residents living near the tank, which is in the 2500 block of Hillside Boulevard, had been put under a “do not drink” advisory after the water utility on Friday morning discovered a lock cut from a…