A 74-year-old woman with major depressive disorder has shown only partial response to sertraline 150 mg daily after 10 weeks of treatment. Her PHQ-9 has decreased from 20 to 14, but she continues to report persistent low mood, poor concentration, and psychomotor retardation. Her medical history includes osteoporosis, mild chronic kidney disease (eGFR 52 mL/min), and controlled hypertension. She has no cognitive impairment on the Montreal Cognitive Assessment (score 27/30), no suicidal ideation, and her metabolic panel and thyroid function are within normal limits. She tolerates sertraline without significant side effects. The PMHNP is planning an augmentation strategy. Which of the following represents the most appropriate approach?