feat(schemas): add initial CUE schema module

[jmgilman]

Summary

• add the nested schemas CUE and Go module with core and IncusOS provider packages
• generate Go types side-by-side from CUE and add embedded schema loading helpers
• wire the repo-local CUE Proto plugin and Moon schema validation tasks into root:check

Testing

• go test ./...
• cd schemas && go test ./...
• cd schemas && proto run cue -- mod tidy --check
• cd schemas && proto run cue -- vet -c=false ./...
• moon run root:check
• git diff --check

Notes

• Removed the temporary functional test command and scratch YAML fixtures before commit.