SAN FRANCISCO (Reuters) - A retired FBI (news - web sites) intelligence expert linked to a spy scandal colored by sex and betrayal has resigned as the top security official at a nuclear research lab as officials there review his work for possible security breaches, an official said on Friday, The resignation follows the arrest on Wednesday of Chinese-American Katrina Leung, charged with serving as a double agent who passed classified national security information to China obtained during a secret 20-year love affair with her FBI handler, James Smith. Court papers refer to a third person, a former San Francisco-based...