Former Congressman John Burton has filed papers allowing him to officially begin organizing a run for San Francisco Mayor-elect Art Agnos' Assembly seat. "This allows me to start raising money" for a campaign, the Democrat said after filing preliminary papers to organize a campaign committee with the city's registrar of voters. Burton, 55, held the Assembly seat from 1965 until 1974, when he won election to the House. He resigned from Congress in 1982, citing personal problems. He plans to make a formal declaration of his candidacy next month. Agnos, elected Dec. 8, takes office Jan. 8. Gov. George Deukmejian will set the primary election date after Agnos' seat officially becomes vacant. The Assembly district covers the east side of San Francisco. Its voter registration is overwhelmingly Democratic. The only other candidate who has filed papers is attorney Roberta Achtenberg, who is making her first try for office.