docs(agents): 补充 BUG 修复测试优先规范

[DCjanus]

Why

• 为 AGENTS.md 增加统一的缺陷修复方法约束。
• 明确先构造稳定失败测试、再修复问题的默认实践，降低无效修复和回归风险。

What

• 在 3.1 长期质量 中补充 test-first bug fixing 规范。
• 要求先添加可稳定复现问题的失败测试，修复后让测试通过，并将其保留为 regression test。

Testing

• 未运行自动化测试；本次仅更新文档。

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: e22aad00-22b1-41af-91d9-84c2a4a24a20

📥 Commits

Reviewing files that changed from the base of the PR and between 06db5ac and aa114a0.

📒 Files selected for processing (1)

• AGENTS.md

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📝 Walkthrough

Summary by CodeRabbit

• Documentation
  • Updated development guidelines with new best practices for bug-fixing procedures.

Walkthrough

The pull request adds a new test-first bug-fixing guideline to the AGENTS.md file under section 3.1 "Long-term quality," recommending developers add failing tests that reproduce bugs before fixing issues and retain them as regression tests.

Changes

Cohort / File(s)	Summary
Documentation AGENTS.md	Added guideline for test-first bug fixing under long-term quality section (3.1).

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Possibly related PRs

• docs(agents): 重构 AGENTS.md（结构化与去冗余） #25: Restructures the AGENTS.md document sections that overlap with this PR's addition of the "3.1 Long-term quality" test-first bug-fixing guideline.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: adding a test-first bug fixing guideline to the AGENTS.md documentation.
Description check	✅ Passed	The description is directly related to the changeset, clearly explaining the motivation, changes made, and testing approach for adding bug-fix testing guidelines.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch docs/add-bug-fix-testing-guideline

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