Add GitHub Actions CI for PRs and align test script with node:test

[Copilot]

This introduces a GitHub Actions CI workflow so pull requests are gated by the existing test suite before merge. It also aligns the repository test command with the actual test file layout so the same command works locally and in CI.

• CI workflow

  • Adds .github/workflows/ci.yml
  • Runs on pull requests, pushes to master, and manual dispatch
  • Executes the test suite across Node.js 20, 22, and 24
  • Uses npm dependency caching via actions/setup-node

• Test command

  • Updates npm test to target the repository's actual test files
  • Replaces the directory-based invocation that fails under current Node.js behavior with an explicit glob

{
  "scripts": {
    "test": "node --test test/*.test.js"
  }
}