infra: set up GitHub Actions CI (build + test on Node 22.x)

[TheStack-ai]

Problem

No CI is configured. PRs can merge without automated verification of the 131-test suite.

Proposed approach

Add .github/workflows/ci.yml that runs on push and pull_request:

• Matrix: Node.js 22.x and 22.5.x — exercises both the better-sqlite3 fallback path and the native node:sqlite path.
• Steps: checkout → setup-node → npm install → npm run build → npm test.

Win-win

• Contributor: simple, visible first PR. Common pattern, good for portfolio.
• Project: regression safety net. Enables confident merges.

Notes

• No secrets needed — tests are fully local.
• The project has 131 passing tests to gate on.
• Consider caching ~/.npm for speed.