ADR: Add hooks alternative, context compaction discussion, and PR feedback

- Add Option 3: ContextHooks with before_run/after_run pattern
- Add detailed pros/cons for both wrapper and hooks approaches
- Add Open Discussion section on context compaction strategies
- Clarify response_messages is read-only (use AgentMiddleware for modifications)
- Add SimpleRAG examples showing input-only filtering
- Clarify default storage only added when NO middleware configured
- Add RAGWithBuffer examples for self-managed history
- Rename hook methods to before_run/after_run

Author: eavanvalkenburg