NEW YORK (Reuters) - Stocks ended the worst week in a year as time runs out on Washington to reach agreement before the government loses its ability to borrow money. The S&P 500 fell every day this week and was down 3.9 percent for the week as legislators failed to work out an agreement to raise the federal borrowing limit, which expires on Tuesday. Investors also worry about the likelihood of a U.S. credit downgrade. The CBOE Market Volatility Index <.VIX>, a gauge of investor fear, jumped as much as 9 percent to its highest level since mid-March before paring...