Add CI/CD pipeline (build + lint + test)

[vayungodara]

Problem

No automated build, lint, or test workflows exist. The only GitHub Actions are Claude code review. This means broken builds and regressions can reach main undetected.

What to change

• Create .github/workflows/ci.yml with a workflow that runs on push to main and on PRs
• Steps: checkout, setup Node, install deps, run next build, run next lint, run e2e tests
• Use Node 20, cache node_modules for speed

Acceptance criteria

• CI runs on every push to main and every PR
• Build failure blocks merge
• Lint errors block merge
• e2e tests run (even if suite is small for now)

Priority: High