For fans of classical radio in the Bay Area, tuning in to KDFC during the early hours of the day has long meant that the musical offerings will come with a distinctive flavor. That inviting quality — folksy, mellifluous, almost preposterously agreeable — is the trademark of Hoyt Smith, the station’s longtime morning DJ. But later this month, after nearly 17 years on the job and a career in radio spanning more than four decades, Smith is planning to walk out of the studio and into a well-earned retirement.