A 62-year-old woman with ischemic cardiomyopathy (ejection fraction 28%) and chronic kidney disease stage 3b (eGFR 38 mL/min/1.73 m²) is on lisinopril 10 mg daily, metoprolol succinate 190 mg daily, and furosemide 40 mg daily. She remains symptomatic with NYHA class III dyspnea. Her current labs show: serum creatinine 1.8 mg/dL, potassium 4.2 mEq/L, BNP 450 pg/mL. Which of the following agents should be added to her regimen to improve mortality and reduce hospitalizations?