Phase 4: GitHub Actions CI

[ChantelRM]

Suggested pace: ~2 sessions this week -- highest priority if time runs short

• Add .github/workflows/ci.yml that installs R + dependencies on push
• Run the testthat suite from Phase 3 in the workflow
• Run the batch pipeline script itself in the workflow (confirms it doesn't just pass tests, it actually executes end to end)
• Add the resulting status badge to the top of the README
• Confirm a deliberately broken push actually fails the workflow (sanity check that it's really running, not a false-green pass)