By GINIA BELLAFANTE

Some ex-prisoners are lucky to get a job assembling Happy Meals at McDonald's. Others, particularly media darlings with good agents, can do a bit better. Coming soon: newly freed murderer/author JEAN HARRIS to the masthead of Lear's magazine, the swank life-style manual for the fiftysomething set. As a contributing editor, she'll write about poverty, prison reform and other social issues, "subjects of great interest to our readers," says senior editor David France. Thankfully, Harris won't be doling out advice to the lovelorn.