Add optional hook callbacks to AgentConfig for cross-cutting concerns
(guardrails, logging, token budgets) without modifying framework internals.
- beforeRun: receives prompt + agent config, can modify or throw to abort
- afterRun: receives AgentRunResult, can modify or throw to fail
- Works with all three execution modes: run(), prompt(), stream()
- 15 test cases covering modify, throw, async, composition, and history integrity
JackChen