chore: make GitHub Pages CounterProof-native

[hippoley]

The Pages deployment still carried SkillFactory-era behavior even though the public surface is now CounterProof Proof Lab.

This change removes that coupling:

• scripts/build_pages.py no longer loads, validates, or serializes skills/**;
• the frontend reads a tiny data/build.json only for the commit badge;
• Pages no longer redeploys when unrelated legacy skills change;
• the workflow is renamed to Deploy CounterProof Proof Lab;
• the static site is always built and validated, even when Pages is not enabled yet;
• only the final configure/upload/deploy steps are skipped when the repository-level Pages switch is still off.

This does not change proof semantics or product behavior; it makes the public deployment path match the current project boundary.