A 62-year-old woman with a 3-week history of cough and fever presents with a left-sided pleural effusion. Pleural fluid analysis shows: LDH 380 IU/L, protein 3.8 g/dL, glucose 35 mg/dL, pH 7.1, and positive Gram stain showing Gram-positive cocci in clusters. What is the most common organism causing empyema in this clinical presentation?