A 52-year-old woman with rheumatoid arthritis on methotrexate and adalimumab presents with acute onset of left shoulder pain, fever (39.2°C), and inability to move the shoulder for 2 days. She denies recent trauma or injection. On examination, the shoulder is warm and swollen with severe pain on all passive movements. Synovial fluid obtained by arthrocentesis shows: WBC 62,000/μL (92% neutrophils), glucose 22 mg/dL (serum 105 mg/dL), protein 5.8 g/dL, Gram stain shows Gram-positive cocci in clusters, and culture is pending. Which organism is most likely and what is the most appropriate antibiotic regimen?