A 32-year-old male from Delhi presents to the de-addiction centre with a 6-year history of heroin use. He reports using 2–3 grams daily via inhalation ("chasing the dragon"). He has attempted self-withdrawal three times but relapsed within 2 weeks each time due to severe anxiety, insomnia, and body aches. On examination, he is alert, with dilated pupils and mild tremor. His vital signs are stable. He is keen on medication-assisted treatment. Which of the following is the most appropriate pharmacological approach for long-term management of his opioid use disorder?