ci: enhance PR review with severity classification and checklist

[qin-ctx]

Description

增强 PR 自动审查的指令，添加严重程度分类和审查 checklist，提升审查质量和深度。

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• 添加 pr_reviewer.num_code_suggestions: 4，增加代码建议数量
• 添加 pr_reviewer.extra_instructions，注入审查标准：
  • [Bug]（必须修复）和 [Suggestion]（建议改进）分级分类
  • 四个审查维度：控制流、数据流、集成点、并发
  • 性能问题检测（N+1 查询、循环内重复计算、大对象拷贝等）
  • 要求引用具体变量名/函数名，附带修复代码块
  • 自动匹配 PR 语言（中文/英文）

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

合入后在新 PR 上验证审查效果。

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• My changes generate no new warnings

[CLAassistant]

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[github-actions]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 1 🔵⚪⚪⚪⚪

🧪 No relevant tests

🔒 No security concerns identified

⚡ No major issues detected

[github-actions]

PR Code Suggestions ✨

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