A 4-year-old boy from rural Maharashtra presents with his mother complaining of painful fluid-filled blisters on his left knee and shin for 3 days. The lesions began as small red papules that rapidly progressed to flaccid bullae with honey-crusted erosions. He attends a local school and shares a dormitory with other children. On examination, the bullae rupture easily, leaving a moist, eroded base with a characteristic golden-yellow crust. The surrounding skin is mildly erythematous but not indurated. Gram stain of the blister fluid shows gram-positive cocci in clusters. What is the most likely causative organism and the first-line antibiotic for this condition?