feat(conductor): Plan-and-Execute 패턴 - Central Planner + Tier executor (env AGENT_MODE)

feat(observability): LangSmith 통합 (wrap_openai + @traceable on 4-Tier agents)

feat(agents): Tier 3 Impact + Tier 4 Response를 tool-using agent로 전환

feat: Tier 4 대응 권고 LLM+RAG 구현

chore: FabAgent 골격 구성