Replace direct directory access with git clone/pull approach:
- Add git configuration options (url, branch, working_dir) to NixOS module
- Update SystemConfig and AgentCommand to use git parameters
- Implement ensure_git_repository() method for clone/pull operations
- Agent clones nixosbox to /var/lib/cm-dashboard/nixos-config
- Maintains security while solving permission denied issues
The agent now manages its own copy of the configuration without
needing access to /home/cm directory.
Christoffer Martinsson
b3c67f4b7f