Add GitHub Actions CI

[richardbowman]

Summary

• Adds a GitHub Actions CI workflow at .github/workflows/ci.yml
• Runs npm test (Jest) on Node.js 20 for every push and pull request targeting master
• Uses npm ci with npm caching for fast, reproducible installs

Why

Ensures all tests pass automatically on every push and PR, providing a safety net against regressions without requiring manual test runs.

Test plan

• Confirm the workflow file is valid YAML
• Verify the CI job triggers on a push to master
• Verify the CI job triggers on a PR targeting master
• Confirm npm test (Jest) runs and passes in the workflow

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