Publish Codecov coverage for README badge

[Copilot]

Adds a coverage badge to the README and ensures CI uploads coverage data so the badge reflects actual test results.

• README badge
  • Add Codecov coverage badge next to the existing unit test badge.
• CI coverage publishing
  • Emit coverage.xml and upload it to Codecov using a repository secret.

- name: Test agrf with coverage
  run: pytest --cov=agrf --cov-report=xml
- name: Upload coverage to Codecov
  uses: codecov/codecov-action@v4
  with:
    files: coverage.xml
    token: ${{ secrets.CODECOV_TOKEN }}

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[ahyangyi]

No, codecov is a bad idea. Why do I need to pay the total control of my Github account for a little badge lol.