Function on_enter 

pub fn on_enter(agent: Arc<dyn TextAgent>, mode: AgentMode) -> StepAction

Build a step-enter action that runs agent in mode when the step first activates. Pair with FlowMonitor::on_enter.

let mon = FlowMonitor::new(flow, Enforcement::Enforce)
    .on_enter("check", on_enter(availability_agent, AgentMode::Dispatch));