/// Observability events emitted during a multi-agent discussion.
///
/// `AgentEvent` is a first-class output of the kernel (observability-first principle).
/// Callers receive a full stream of events via `mpsc::Sender<AgentEvent>`.
///
/// Phase 1 has exactly 6 variants. `Evolved` is a Phase 2 variant added in the
/// independent agent-kernel repository.