SAN DIEGO — San Diego police have created a team of investigators focused on curbing the proliferation of ghost guns in the city and targeting people who are making and selling the untraceable weapons, department officials said Wednesday. Police Chief David Nisleit also announced a new plan aimed at reducing violent crime, which has been on the upswing not just locally but in large cities across the country. The approach calls for ratcheting up policing efforts where shootings have happened, including an increased presence in the Gaslamp Quarter and beyond, the site of recent shootings — including a July 4 gun battle in which two people were shot. One of them was hospitalized with life-threatening injuries. Police said at least 35 bullets were fired from at least three guns.