PLANO, Texas — Alliance Data Systems has announced that its Columbus-based card-services business has signed an agreement to provide private label, co-brand and commercial credit-card services for Signet Jewelers.   Signet, with headquarters in Akron, is the world's largest retailer of diamond jewelry, operating approximately 3,600 stores primarily under the name brands of Kay Jewelers, Zales, Jared The Galleria Of Jewelry, H.Samuel, Ernest Jones, Peoples and Piercing Pagoda.   As part of the deal, Alliance Data will acquire prime-only credit quality accounts with a value of approximately $1 billion in receivables from Signet’s existing credit card portfolio. Alliance Data will also acquire a portion of Signet’s existing customer care operations in Ohio, including approximately 250 employees.