A 32-year-old woman with major depressive disorder has been on sertraline 100 mg daily for 3 months with good response. She presents to the emergency department with a 6-hour history of agitation, confusion, and tremor. On examination: temperature 38.8°C, heart rate 118/min, blood pressure 148/92 mmHg, hyperreflexia, and clonus in the lower limbs. She denies recent medication changes but admits to self-starting fluoxetine 20 mg daily 2 days ago for "faster relief." Creatine kinase is 1200 U/L (normal <200). What is the most appropriate immediate management?