Teresa Heinz Kerry says she’ll step down as chairwoman of the Heinz Endowments and her three sons will fill the position in a series of four-year rotations starting this fall. The plan was released Monday after two years of discussions with the foundation's board of directors and its top executive. It was unanimously approved by the 14-member board. Heinz Kerry, 77, plans to step down in October to pave the way for her sons. …