chore: add minimal CI

[devrodri-com]

Summary

• Add a minimal GitHub Actions CI workflow.
• Run on pull_request and pushes to main.
• Use Node 20, npm ci, and npm run build.

Validation

• npm ci
• npm run build
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ci.yml"); puts "ci.yml OK"'\n\n## Notes\n- Lint is omitted because package.json does not define a lint script.\n- Tests are omitted because package.json does not define a test script.\n- No secrets/env vars were added, read into CI, or modified.\n- No deploy/Vercel config changed.

[vercel]

Deployment failed with the following error:

Resource is limited - try again in 1 day (more than 100, code: "api-deployments-free-per-day").

Learn More: https://vercel.com/softbms-projects?upgradeToPro=build-rate-limit