ci: add GitHub Actions workflow for PR checks

[greg3d]

Summary

Adds a minimal GitHub Actions workflow at `.github/workflows/ci.yml` that runs the mocha suite on every push to master and every PR against master.

Setup

• Matrix: Node 18, 20, 22
• Steps: `actions/checkout@v4` → `actions/setup-node@v4` → `npm install` → `npm test`
• `fail-fast: false` so all matrix legs report independently

Why this minimal shape

• No lockfile in the repo (intentional for a library), so no `cache: npm` and no `npm ci`.
• 9 mocha specs, ~1 s suite — no need for parallelism, sharding, or caching.

Follow-ups (out of scope)

Once green, this workflow can be marked as a required status check in the repo branch protection settings.

Test plan

• Workflow runs successfully on this PR (all three matrix legs green)

[Copilot]

Pull request overview

Adds a minimal GitHub Actions CI workflow to run the project’s Mocha test suite for pushes to master and PRs targeting master, ensuring basic automated checks run consistently across supported Node versions.

Changes:

• Introduces a CI workflow triggered on push/pull_request for the master branch.
• Runs npm install and npm test across a Node version matrix (18, 20, 22) with fail-fast: false.

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[greg3d]

Superseded by #3 — consolidated into a single PR per request. The workflow lives on publish-readiness now, upgraded to use npm ci + cache: npm against the committed lockfile.