docs: add FDE CLI quickstart

[namastex888]

Summary

• add a repo-level FDE CLI quickstart for installing @khal-os/app-kit from npmjs, logging in, scaffolding, dry-running install, and proof commands
• cross-link the quickstart from getting started, local dev, environment ladder, publish, and customer install docs
• remove stale guidance that app repos/packs should be handed off as GitHub Packages/npm artifacts; document git-native app install instead

Verification

• python3 -m json.tool docs.json
• custom docs.json/internal-link checker
• git diff --check
• npx -y mint@latest broken-links