One of California's best-known political strategists, attorney John Emerson, has gone to work for Los Angeles City Atty. James K. Hahn as his $95,700-a-year executive assistant. Emerson, most recently a top adviser in former Colorado Sen. Gary Hart's Democratic presidential campaign, will handle day-to-day office operations. He is also expected to be a political adviser to Hahn, who is often mentioned as a candidate for mayor or other offices. Before joining the Hart campaign, Emerson was a member of the politically influential Los Angeles law firm of Manatt, Phelps, Rothenberg and Tunney.