Add a smoke test workflow to validate repo structure on push

Overview

As a basic sanity check, we want a GitHub Actions workflow that verifies key files exist in
the repo whenever someone pushes to main. Simple way to catch accidental deletions of
important files like README or CONTRIBUTING.

Steps

1. Create .github/workflows/repo-structure-check.yml
2. Set it to trigger on push to main
3. Add steps that check for the existence of README.md, CONTRIBUTING.md, and .gitignore
4. Make sure the workflow fails with a clear message if any file is missing

Acceptance Criteria

• .github/workflows/repo-structure-check.yml added
• Workflow triggers on push to main
• Workflow passes on current repo state
• Fails with a helpful message if a required file is missing

Resources

• GitHub Actions workflow syntax