Two days after being promoted to the new position of Target Corp’s Chief Operating Officer, John Mulligan’s big job is clear: fix a supply chain that the company blamed for shortages of goods that are undermining sales growth. On Wednesday, the fourth-largest U.S. retailer blamed its “incredibly complex supply chain” for unacceptable stock levels at its stores this year, forecasting weak growth in current quarter sales at established stores. The focus on the supply chain network is the latest move by CEO Brian Cornell, who took charge a year ago and announced a restructuring plan in March.