A 48-year-old man with major depressive disorder has completed two adequate antidepressant trials — fluoxetine 60 mg for 10 weeks and venlafaxine XR 225 mg for 8 weeks — with less than 25% improvement in symptoms. His current PHQ-9 score is 19. He has no history of bipolar disorder, no psychotic features, and his renal function and thyroid panel are within normal limits. He is currently taking venlafaxine XR 225 mg daily. The PMHNP is considering augmentation. Which augmentation strategy has the strongest evidence base for treatment-resistant depression in this clinical scenario?