Adds macOS sandboxing support similar to Linux bubblewrap implementation:
- Created clan_lib/sandbox_exec module with sandbox profile creation
- Implemented file system isolation allowing only tmpdir and nix store access
- Added network restrictions (deny outbound except localhost)
- Integrated sandbox-exec command into vars generation on macOS
- Added comprehensive test suite for macOS sandbox functionality
- Fixed working directory handling for generators writing to CWD
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