Add GitHub Actions CI to run build + tests

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Add a GitHub Actions workflow to run build and tests.

Add .github/workflows/ci.yml with a job that:

• Runs on push and pull_request.
• Checks out the repo, sets up Node.js 24.x, runs npm ci, npm run build, and npm test.

Acceptance criteria:

• Workflow file added and passes basic syntax checks.

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.