A 32-year-old man from Delhi presents with a 3-day history of fever (39.2°C), severe right knee pain, and swelling. He has a history of intravenous drug use. On examination, the knee is warm, erythematous, and tender with restricted movement. Synovial fluid analysis shows WBC 45,000/μL (90% neutrophils), protein 6.2 g/dL, glucose 18 mg/dL (serum glucose 110 mg/dL), and Gram stain reveals Gram-positive cocci in clusters. Blood culture is pending. Which organism is most likely responsible, and what is the most appropriate initial antibiotic therapy?