Event types, [EventActions], and the [Hook] system for the behest agent runtime.

This crate provides:

• [AgentEvent]: the canonical 17-variant event covering the full agent lifecycle (moved here from behest::runtime::event).
• [EventActions]: side-effect declarations that accompany events.
• [Hook]: single-method observer that returns Vec<EventActions>.
• [HookStack]: ordered dispatch of multiple hooks.

Design

Hooks use a single-method pattern (inspired by Rig): one on_event() method receives an [AgentEvent] and returns Vec<EventActions>. Adding new event types never requires changing the Hook trait signature.