Replace the repo + default.xml + setup-environment workflow with kas:
- add kas/forgefirm-glowforge.yml (Scarthgap layers pinned, meta-openglow
as a local sibling) and kas/README.md
- remove default.xml, base/conf/forgefirm-bblayers.conf,
scripts/setup-environment
- .gitignore the kas-managed layers/ and build caches
- BUILD.md: kas host setup and build instructions
