Congress began work on the 1990 budget in uncharacteristic bipartisan spirit but top senators conceded their spending plan is only a first step that leaves deeper cuts to the future. On a 16-7 vote, the Senate Budget Committee approved a $1.16 trillion blueprint for the fiscal year that begins Oct. 1, a document implementing the deficit-reduction pact reached last week between the Bush administration and congressional leaders. Nine Democrats and seven Republicans supported the proposal. The plan touts enough cuts to bring next year's deficit to $99.4 billion, less than the Gramm-Rudman law's $100 billion ceiling.