Two days after being promoted to the new position of Target Corp’s (TGT.N) 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.