Rewrites just clean/nuke to remove only ignored build artifacts (.DerivedData/.build), never git-checkout or rm tracked project files (previously could destroy uncommitted work). Adds a CI guard (check-just-clean-safety.sh) against reintroducing source-mutating cleanup, plus README/Local.xcconfig modernization.
- Add check for full Xcode vs command line tools only
- Provide clearer error messages with setup instructions
- Verify Xcode is installed and properly configured
- Add better validation for development environment
Fixes issue where 'just run' fails with cryptic error when only
command line tools are installed. Now gives clear guidance on
installing full Xcode and configuring xcode-select properly.
Resolves#760
