General Motors Co. said Monday it would launch a $5 billion share buyback in an effort to avert a proxy war with dissident investors, and detailed a plan that could allocate investors even more cash. GM said it had reached a deal with an investor group that averts a proxy fight over its balance sheet and governance. As part of the agreement, investment-group-leader Harry Wilson will drop his effort to get a seat on GM’s board. GM shares were up 2.4 percent at $37.42 in afternoon trade.