A 58-year-old man from rural Maharashtra presents with a 3-month history of altered bowel habits, abdominal pain, and weight loss. On examination, a palpable mass is felt in the left lower abdomen. Colonoscopy reveals a stenosing lesion in the sigmoid colon with biopsy confirming adenocarcinoma. CT abdomen shows the tumour invading the muscularis propria with no distant metastases, but 2 out of 12 regional lymph nodes are involved. What is the most appropriate next step in management?