[WIP] ci: add automated PR review workflow using Doubao model

[qin-ctx]

Description

添加基于 pr-agent 的自动 PR 审查 workflow，使用豆包 Doubao-Seed-2.0-Code 模型，在 PR 创建/更新时自动进行代码审查和改进建议。

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

• 新增 .github/workflows/pr-review.yml，配置 qodo-ai/pr-agent
• PR 创建/更新时自动触发 review 和 improve
• describe 默认关闭，可通过评论 /describe 手动触发
• 使用火山方舟 API 对接 doubao-seed-2-0-code-preview-260215 模型

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
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

需要在仓库 Settings -> Secrets 中配置 DOUBAO_API_KEY 后，后续 PR 才能验证效果。

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• My changes generate no new warnings
• I have made corresponding changes to the documentation
• Any dependent changes have been merged and published

Additional Notes

合并后需要在仓库 Settings -> Secrets and variables -> Actions 中添加 DOUBAO_API_KEY（火山方舟 API Key），否则 workflow 会因缺少密钥而失败。

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