An FBI agent who mistakenly shot an unarmed Pasadena man in the face last year says in court papers that he believed the unwitting victim was a wanted bank robber who was reaching for a weapon when, "in a split-second decision," the agent fired "a single shot for his and his fellow officers' safety." Special Agent Christopher R. Braga offered his first detailed account of the event in court papers made public yesterday as he and two other agents argued that a $10 million federal lawsuit brought by shooting victim Joseph C. Schultz lacks merit and should be dismissed.