[Bug]: Too mush token cost for [Demo PG - GAAP use case ]

### Summary

5,014,326 token cost (about $2) in https://bailian.console.aliyun.com for task
```
datapaw run --datasource-id postgresql-demo-gaap "分析 2026 年 3 月 product X 有效用户的平均 GAAP 值，展示时间趋势，并结合相关 KG 事件解释异常峰值"
```

### Minimal reproduction

minimal demo as README

### Environment

minimal demo as README

### Sanitized logs

```shell

```

### Checklist

- [x] I searched existing issues and discussions.
- [x] I removed secrets, private URLs, and business data.
- [x] This is not a security vulnerability.

Start with the README's minimal demo and the `datapaw run` entry point for `postgresql-demo-gaap`. Trace how the sample query and knowledge-graph explanation consume tokens, then compare the reported usage with the Bailian console for the reproduction command. Done means the demo no longer incurs the reported excessive token cost and the result remains correct.

Written by the indexing model from the issue text.