Add GitHub Actions CI for unit tests

[frarredondo]

Summary

• Closes Add unit test CI check on pull requests #17: add .github/workflows/ci.yml with job test on pull requests and pushes to main.
• Installs uv sync --extra dev only (no Apple Silicon ML extras; unit tests do not need them).
• Documents CI in README and CONTRIBUTING.

Test plan

• Local uv run pytest green on this branch
• Confirm the test check runs on this PR
• After merge, optionally add test as a required status check on Protect main

[frarredondo]

Babysit: CI test failed on test_cli_format_feed because Rich soft-wrapped the path (feed-a-e / p.json). Merged latest main (#16) and tightened the assertion to check ok/failed counts + output file instead of an unbroken basename in stdout.

[frarredondo]

Added Codecov + coverage ratchet on top of CI:

Measured baseline (unit suite, no ML extras):

• Statements / lines: ~65% → ratchet floor ≥ 65% (scripts/check_coverage_ratchet.py)
• Branches: ~46% (reported, not gated)
• Combined Cover column: ~60%

Also: README Codecov + CI badges, codecov.yml (informational statuses), pytest-cov in dev.

Action needed from you: create the project on codecov.io, add repo secret `CODECOV_TOKEN`. Upload step uses `fail_ci_if_error: false` so missing token won’t fail CI; the ratchet still enforces coverage.

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

ℹ️ You can also turn on project coverage checks and project coverage reporting on Pull Request comment

Thanks for integrating Codecov - We've got you covered ☂️

[frarredondo]

COVERAGE_RATCHET_MIN is now a repo Actions variable (set to 65). CI passes it into the ratchet script; local default remains 65 if unset. Also set Codecov fail_ci_if_error: true now that CODECOV_TOKEN exists.