Implement git clone approach for nixos-rebuild

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.

agent/src/communication/mod.rs

@@ -106,7 +106,9 @@ pub enum AgentCommand {
     },
     /// Rebuild NixOS system
     SystemRebuild {
-        nixos_path: String, // Path to nixosbox directory
+        git_url: String,
+        git_branch: String,
+        working_dir: String,
     },
 }