A 38-year-old woman presents to the emergency department with a 3-month history of progressive fatigue, weight loss of 8 kg, and recurrent episodes of syncope. On examination, she is hypotensive (BP 88/54 mmHg), has generalized hyperpigmentation over the knuckles, elbows, and buccal mucosa, and appears chronically ill. Laboratory investigations reveal: sodium 128 mEq/L, potassium 5.8 mEq/L, fasting glucose 62 mg/dL, and elevated ACTH 850 pg/mL with cortisol 2.2 µg/dL (8 AM). A chest X-ray shows bilateral hilar lymphadenopathy. What is the most likely diagnosis?