SAN DIEGO — Pitcher Jon Lester agreed Tuesday night to a $155 million, six-year contract with the Cubs, the first big deal of the offseason involving a top-level starting pitcher and one Chicago hopes will help end more than a century of frustration at Wrigley Field’s Friendly Confines. Lester’s contract, agreed to on the second day of baseball’s winter meetings, contains an option for 2021 that, if it becomes guaranteed, would make the deal worth $170 million over seven seasons.