ANNA Wintour is losing her event planner of 11 years today. Stephanie Winston Wolkoff has resigned as director of special events for the fashion bible after more than a decade as Wintour's right-hand woman, a role in which she's organized the increasingly commercial Costume Institute Ball at the Metropolitan Museum each year. The granddaughter of jeweler Harry Winston, who married real-estate developer David Wolkoff in 2000, says she's leaving to spend more time with him and their three kids. -PAGE SIX, NY Post