A 27-year-old woman with severe OCD has completed 16 sessions of exposure and response prevention therapy and has been on fluvoxamine 300 mg daily for 14 weeks with confirmed adherence. Her Yale-Brown Obsessive Compulsive Scale score has decreased from 32 to 26, representing only a partial response. She remains significantly impaired by contamination obsessions and handwashing rituals that consume 4 hours daily. What is the most appropriate augmentation strategy?