A 48-year-old man with treatment-resistant depression has been switched from fluoxetine to venlafaxine XR 225 mg daily. After 3 weeks, he develops severe headache, neck stiffness, tremor, hyperreflexia, and agitation. His core temperature is 38.5°C. He admits to taking an herbal supplement containing St. John's wort for 2 weeks alongside the venlafaxine. What is the most likely diagnosis and the primary mechanism of toxicity?