The Mets held a groundbreaking ceremony Monday to celebrate the 45,000-seat ballpark that will replace Shea Stadium at the start of the 2009 season. The $800 million stadium will be called CitiField, part of a 20-year sponsorship deal between the Mets and Citigroup Inc. that is said to be worth an average of $20 million annually. Mets owner Fred Wilpon declined to provide details on the financial arrangement. The design is reminiscent of Ebbets Field, the storied home of the Brooklyn Dodgers.