A 32-year-old woman presents with fatigue, weight loss, hyperpigmentation of the skin, dizziness on standing, and recurrent vomiting for 6 months. On examination, her blood pressure is 90/60 mmHg while supine and 70/50 mmHg on standing. Laboratory investigations show: Serum Na⁺: 124 mEq/L Serum K⁺: 6.2 mEq/L Blood glucose: 58 mg/dL Serum cortisol: Low Plasma ACTH: High What is the most likely diagnosis?