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.

dashboard/src/config/mod.rs

@@ -7,6 +7,7 @@ use std::path::Path;
 pub struct DashboardConfig {
     pub zmq: ZmqConfig,
     pub hosts: HostsConfig,
+    pub system: SystemConfig,
 }
 
 /// ZMQ consumer configuration
@@ -21,6 +22,14 @@ pub struct HostsConfig {
     pub predefined_hosts: Vec<String>,
 }
 
+/// System configuration
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SystemConfig {
+    pub nixos_config_git_url: String,
+    pub nixos_config_branch: String,
+    pub nixos_config_working_dir: String,
+}
+
 impl DashboardConfig {
     pub fn load_from_file<P: AsRef<Path>>(path: P) -> Result<Self> {
         let path = path.as_ref();