What is the first-line treatment for a sinus tract in an otherwise healthy patient?

Definitive root canal treatment of the source tooth (cleaning, shaping, and obturating the canal system to eliminate the infection) is the correct first-line treatment — once the source of infection is removed, the sinus tract resolves on its own without needing surgical incision/drainage or antibiotics in an otherwise healthy patient. Debridement alone (without completing definitive RCT) would be incomplete treatment.