Wells Fargo, the largest bank on the West Coast, agreed to buy Wachovia Corp. for $15.1 billion in stock, creating one of the worldÔÇÖs biggest banks and defeating Citigroup Inc.ÔÇÖs offer four days ago for part of the lenderÔÇÖs assets. ┬á Citigroup, the biggest bank in terms of assets by mid-year said late last month that it planned to buy WachoviaÔÇÖs operations for about $2.16 billion. The transaction had been supervised by the Federal Deposit Insurance Corp. and assisted by the government.