Enhance context handling and clarify agent interactions

[LienJack]

• Updated the README.md to include support for an optional contextEnvelope in request bodies, improving the clarity of context management.
• Refactored agent types to incorporate ContextEnvelope, GraphEffectiveContextSummary, and GraphContextConflictHint, enhancing type safety and modularity.
• Improved the GraphBuilderService to build and include context evidence in SQL run traces, allowing for better context differentiation in responses.
• Enhanced the ClarifyNode to utilize context information for more accurate clarification prompts, improving user interaction.
• Introduced a new Slot Filling Context module to streamline the decision-making process for missing critical slots in user queries.
• Updated various services and controllers to support the new context envelope structure, ensuring consistent handling across the application.

This commit strengthens the agent's ability to manage context effectively, leading to improved user experience and more accurate query handling.

Summary

• 简述本次改动范围与目标

Frontend Checklist

• 仅使用 React + shadcn-ui 体系，未引入未批准 UI 框架
• 业务组件未使用常规内联样式对象（style={{...}}）
• 覆盖并验证了关键状态（loading/empty/error/success）
• 核心链路（会话创建、消息发送、SQL 预览）无回归
• 已补充或更新必要测试

Validation

• pnpm --filter @text2sql/frontend lint
• pnpm --filter @text2sql/frontend test
• pnpm --filter @text2sql/frontend build

Post-Deploy Monitoring & Validation

• No additional operational monitoring required（前端展示层改造，无后端运行时逻辑变化）