refactor(install): skip upstream tests by default

[Terry-Mao]

Summary

• Do not install upstream-managed .github/aicodingflow-tests into target repositories by default.
• Keep installer tests and product docs aligned with the narrower sync scope.

Validation

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/tests -p 'test_install_script.py'
• git diff --check

[github-actions]

已检查安装脚本、测试断言和产品文档更新。默认安装不再同步 .github/aicodingflow-tests，与 README/raw/wiki 文档一致；已安装 workflow 未依赖该目录，安全审查未发现新增风险。