docs(readme): add zero-key quickstart

[xianjianlf2]

Summary

• add a copy-pasteable offline quickstart immediately after installation
• use the built-in mock LLM and retriever with a minimal dataset
• show the full dataset → retrieval → generation → metrics → report path before API-key setup

Verification

• copied the documented files into a clean directory
• oaeval validate quickstart.yaml passed with no API keys
• oaeval run quickstart.yaml completed with 1 item and 0 errors
• uv run pytest tests/integration/test_pipeline/test_mock_e2e.py (3 passed)

Repository-wide Ruff, mypy, and pytest currently fail on pre-existing main-branch issues (36 lint errors, 163 type errors, and missing numpy during test collection); this README-only change does not touch those paths.

Closes #331

Implementation and validation were assisted by OpenAI Codex; I reviewed the final documentation and executed the snippet end-to-end.

[github-actions]

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