Ventura County reached a tentative agreement with its largest employee union, ending five months of negotiations and averting a strike threatened for today. Details of the accord were not released. Officials of the Public Employees Assn. of Ventura County will explain the terms to their 2,900 members today before making a public announcement. County negotiator Ed McLean said only that the contract met guidelines imposed by the Ventura County Board of Supervisors. Final approval is expected to take about 10 days and is contingent upon ratification by workers and supervisors.