chore: tighten setup verification

[Programcode]

Summary

• replace stale wrapper-command integration test with a clean-environment smoke test for the supported installer/doctor/version/Python API contract
• document the verification checklist in CONTRIBUTING.md
• apply ruff import/style cleanup and small mypy fixes so lint, typing, tests, smoke test, and package build all pass

Verification

• ruff check agent_reach tests
• mypy agent_reach
• python -m pytest tests/ -q
• bash test.sh
• python -m build

[Panniantong]

感谢加强安装验证。merged #577 已把 test.sh 改为干净隔离环境、真实 CLI 安装、只读 Doctor 和完整 pytest，并让安装失败非零退出，因此关闭。