McKinsey & Co. are supposed to know it all. They’re business consultants who travel the world and charge corporations top dollar to help them run with state-of-the-art efficiency. McKinsey gets called in when things are good (time to expand!) or, like now, bad (to give bosses cover to make bloody-minded decisions). Known as “the Firm,” McKinsey has consultants in 90 offices in 50 countries. They hire the best-credentialed people, and they pay them well. By their own count, they serve 80 of the Fortune 100, though who, precisely, retains them, they try to keep confidential. Last week, they were hired to try to fix the fabulously free-spending Condé Nast.