Tony Ridder won’t be entirely out of the newspaper business when Knight Ridder sells its assets and closes on Tuesday. The Merc, quoting unidentified sources, says that the Knight Ridder chairman and chief executive will join the board of McClatchy Co., the Sacramento-based publisher that is buying Knight Ridder’s 32 papers for $4.6 billion. The purchase agreement gives Knight Ridder two seats on the board. Who will fill the other seat is unknown. Ridder will be the second former Mercury News publisher on McClatchy’s board — the other is Larry Jinks, who was publisher from 1989-94.