Enable GitHub Pages for spec deployment

[wollax]

Problem

The Deploy Spec to GitHub Pages CI workflow fails on every push with:

Error: Failed to create deployment (status: 404)
Ensure GitHub Pages has been enabled: https://github.com/wollax/cupel/settings/pages

GitHub Pages is not enabled on the repository, so the actions/deploy-pages@v4 step has nowhere to deploy to.

Fix

1. Go to Settings → Pages on the repo
2. Under Source, select GitHub Actions
3. Save

The workflow already builds the mdBook spec correctly — it just can't deploy the artifact. Once Pages is enabled, the deploy step will succeed and the spec will be served at https://wollax.github.io/cupel/.

Impact

• CI shows a failing workflow on every push (noise)
• The spec mdBook is never publicly deployed