* feat(agent): add beforeRun / afterRun lifecycle hooks (#31)
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
* fix(agent): preserve non-text content blocks in beforeRun hook
- applyHookContext now replaces only text blocks, keeping images and
tool results intact (was silently stripping them)
- Use backward loop instead of reverse() + find() for efficiency
- Clarify JSDoc that only `prompt` is applied from hook return value
- Add test for mixed-content user messages
* fix(agent): address review feedback on beforeRun/afterRun hooks
- Normalize stream done event to always yield AgentRunResult
- Move transitionTo('completed') after afterRun to fix state ordering
- Strip hook functions from BeforeRunHookContext.agent to avoid self-references
- Pass originalPrompt to applyHookContext to avoid redundant message scan
- Clarify afterRun JSDoc: not called when the run throws
- Add tests: error-path skip, outputSchema+afterRun, ctx.agent shape, multi-turn hooks
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