WASHINGTON — Treasury Secretary Janet Yellen told Congress on Friday that she will start taking emergency measures next week to keep the government from an unprecedented default on the national debt, warning that a default would cause “irreparable harm to the U.S. economy and the livelihoods of all Americans.” In a letter to House and Senate leaders, Yellen said her actions will buy time until Congress can pass legislation to either raise the debt limit or suspend it again for a period of time. The debt limit has been suspended for the past two years but will go back into effect on July 31.