A 38-year-old woman from rural Maharashtra presents with a 3-month history of progressive fatigue, weight loss (8 kg), and postural dizziness. On examination, she is hypotensive (BP 95/60 mmHg), has generalized hyperpigmentation over knuckles and oral mucosa, and demonstrates a positive orthostatic vital sign change. Serum sodium is 128 mEq/L and potassium is 5.8 mEq/L. A morning cortisol is 3 µg/dL (normal >10 µg/dL). What is the most appropriate next step in management?