Add GitHub Actions CI (build + test + typecheck)

## Why\ncare-tools currently lacks an enforced CI gate for changes.\n\n## Scope\n- Add workflow for Node install, build, tests, typecheck/lint.\n- Ensure PRs to main require green checks.\n\n## Acceptance Criteria\n- [ ] CI executes on push/PR\n- [ ] Tests + type checks are required for merge\n- [ ] CONTRIBUTING includes one-command local verify\n