ci: pin Node 20 via .nvmrc, add CI badge

[erubboli]

Validation PR for the repo's automation loop:

• pins the Node version (.nvmrc 20) so local dev and CI agree
• adds the CI status badge to the README

Also proves end-to-end: CI workflow green, AI code review posts, and branch protection on main requires review + the build check.

[github-actions]

🔍 OpenCodeReview found 1 issue(s) in this PR.

• ✅ Successfully posted inline: 0 comment(s)
• 📋 Routed to summary by policy: 1 comment(s)

---

📄 .nvmrc (L1-L1)

⚠️ GitHub could not post this as an inline comment: Routed to summary (severity low · category maintainability)

Loose major-version pin ('20') will resolve to the latest 20.x at install time, which may yield nondeterministic builds across machines/CI. Consider pinning a specific version (e.g., '20.11.0') if build reproducibility is required; otherwise '20' is an acceptable convention.