Celebvocates! You never know where they’ll show up, part II. Hours after Bradley Cooper popped up at the White House to hold forth on mental health Monday, Maggie Gyllenhaal slipped into town for her own cause. The Oscar nominee attended a private party Monday night in Washington to announce that she has joined the advisory board of DC Greens, a local healthy-food advocacy nonprofit group promoting school gardens and farmers’ market vouchers for low-income residents. In a statement, she called it “a model for other programs across the country.”