A 45-year-old man on ICU has a ventriculoperitoneal (VP) shunt for hydrocephalus. He develops fever, headache, and reduced GCS. CSF obtained from the shunt reservoir shows: WCC 850/µL (90% neutrophils), protein 2.8 g/L, glucose 1.0 mmol/L (serum 6.5 mmol/L). Gram stain shows Gram-positive cocci in clusters. What is the most likely organism and appropriate antibiotic?