In an agreement being hailed as a national model, the leaders of Maryland, Pennsylvania, Virginia, and Washington, D.C., pledged yesterday to curb sprawl in the Chesapeake Bay area. They vowed to reduce by 30 percent the rate of development in the 64,000-square-mile watershed by 2012, and to permanently protect 1.6 million acres and restore 25,000 acres of wetlands by 2010. The voluntary agreement also aims to restore the bay’s populations of blue crabs and oysters. It gives each jurisdiction the flexibility to determine how and where to curb growth. U.S. EPA Administrator Carol Browner praised the agreement as the first of its kind in the nation to include local, state, and federal governments, and environmentalists were generally supportive as well.