SAN DIEGO — A Mexican man who lied about his citizenship to become a U.S. Border Patrol agent and admitted helping illegal aliens to enter the country was sentenced Friday to five years in federal prison. Oscar Antonio Ortiz, 29, had pleaded guilty to conspiring to bring in more than 100 illegal immigrants, falsely claiming to be a U.S. citizen, lying in order to get a gun, and owning a gun despite having entered the country illegally. "I grew up thinking I was a U.S. citizen," Ortiz told the judge. "I was living the dream, and suddenly I woke up in my prison and now I realize who I am." A defense lawyer asked for two years in prison and a prosecutor for about three, but U.S. District Judge John A. Houston opted for a longer sentence. "You were one of the gatekeepers," the judge said. "Our border security depends on people like you." North County gang detectives investigating drug deals were the first to discover Ortiz's criminal background when they overheard him on