Add CI workflow to run npm test on PRs

[kyroskoh]

Context

Today CI is release-only (.github/workflows/release.yml); unit tests only run via local npm test.

Goal

Every pull request runs the existing test suite before merge.

Acceptance criteria

• GitHub Actions workflow triggers on pull_request (and optionally push to main)
• Workflow runs npm ci / npm install and npm test
• Failures block merge (required check if branch protection allows)
• Documented briefly in README or PRD references

Notes

Mirror the current package.json test script; keep scope to unit tests.

References

• package.json → scripts.test