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/app.rs

@@ -22,6 +22,7 @@ pub struct Dashboard {
     terminal: Option<Terminal<CrosstermBackend<io::Stdout>>>,
     headless: bool,
     initial_commands_sent: std::collections::HashSet<String>,
+    config: DashboardConfig,
 }
 
 impl Dashboard {
@@ -132,6 +133,7 @@ impl Dashboard {
             terminal,
             headless,
             initial_commands_sent: std::collections::HashSet::new(),
+            config,
         })
     }
 
@@ -297,8 +299,10 @@ impl Dashboard {
             }
             UiCommand::SystemRebuild { hostname } => {
                 info!("Sending system rebuild command to {}", hostname);
-                let agent_command = AgentCommand::SystemRebuild {
-                    nixos_path: "/home/cm/nixosbox".to_string(), // Fixed path per requirements
+                let agent_command = AgentCommand::SystemRebuild { 
+                    git_url: self.config.system.nixos_config_git_url.clone(),
+                    git_branch: self.config.system.nixos_config_branch.clone(),
+                    working_dir: self.config.system.nixos_config_working_dir.clone(),
                 };
                 self.zmq_command_sender.send_command(&hostname, agent_command).await?;
             }

dashboard/src/communication/mod.rs

@@ -23,7 +23,9 @@ pub enum AgentCommand {
     },
     /// Rebuild NixOS system
     SystemRebuild {
-        nixos_path: String, // Path to nixosbox directory
+        git_url: String,
+        git_branch: String,
+        working_dir: String,
     },
 }
 

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();