Add pytest-cov coverage reporting to CI

[qiaobochi040726-source]

Fixes #28

• pyproject.toml:dev 依赖加 pytest-cov>=4.0;[tool.coverage.run] source=["stellargate"](omit tests)
• 新增 .github/workflows/test.yml:push/PR 触发,Python 3.10/3.11 matrix,pytest --cov=stellargate --cov-fail-under=60,上传 .coverage 报告
• 覆盖率参数只放 CI,不进 pyproject addopts,本地裸跑 pytest 不受影响

验证:本地实测 Total coverage 62.28%(阈值 60 达标),15 passed。遗留:cli.py 覆盖率为 0%(无 CLI 层测试),后续加测试可上调阈值。