ci: add GitHub Actions workflow for lint, test, and build

[t0kubetsu]

Problem

The project has no CI pipeline. There are no automated checks running on pull requests or pushes. This means:

• Lint regressions (e.g. unused vars, rule violations) go undetected until someone runs npm run lint locally
• Test failures are not caught automatically
• Build breakages can be merged without notice

Expected

Add .github/workflows/ci.yml that runs on every push and pull request to main:

1. npm ci — reproducible install
2. npm run lint — ESLint
3. npm run test:unit -- --run — Vitest unit tests
4. npm run build — production build

The workflow should run on ubuntu-latest with Node.js 24 (matching the Dockerfile).