Improve first-time contributor onboarding in README with enforceable guidance

[Copilot]

This issue asked for a contributor-perspective review and concrete changes that make first contributions faster and safer, without adding new markdown files.
This PR tightens the onboarding path in existing docs and adds a focused guardrail test so the guidance stays intact.

• README: explicit first-PR quick path

  • Added a dedicated Contributing (First PR Quick Path) section to README.md.
  • Included a minimal, actionable command sequence for local setup + pre-PR checks.
  • Added high-signal helper commands for discoverability (make help, make ci).

• README: contributor safety constraints

  • Added explicit no-new-markdown guidance and linked enforcement to tests/test_link_checker.py.
  • Clarified intent (prevent documentation drift) to reduce policy ambiguity for new contributors.

• Test guardrail for onboarding regressions

  • Added tests/test_readme_contributor_onboarding.py.
  • Verifies:
    • onboarding section exists,
    • required contributor commands are present in the intended section,
    • explicit safety-policy wording remains present.

### Contributing (First PR Quick Path)

make setup
make dev-install
make format
make lint
make test

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