Add configuration hash display to system panel

- Collect config hash from cloned nixos-config git repository
- Display "Config: xxxxx" after "Build: xxxxx" in NixOS section
- Uses /var/lib/cm-dashboard/nixos-config directory
- Shows actual configuration hash vs nixpkgs build hash

agent/src/collectors/nixos.rs

@@ -63,6 +63,29 @@ impl NixOSCollector {
         Ok("unknown".to_string())
     }
 
+    /// Get configuration hash from cloned nixos-config git repository
+    fn get_config_hash(&self) -> Result<String, Box<dyn std::error::Error>> {
+        // Get git hash from the cloned nixos-config directory
+        let config_path = "/var/lib/cm-dashboard/nixos-config";
+        
+        let output = Command::new("git")
+            .args(&["log", "-1", "--format=%h"])
+            .current_dir(config_path)
+            .output()?;
+        
+        if !output.status.success() {
+            return Err("git log command failed".into());
+        }
+        
+        let hash = String::from_utf8_lossy(&output.stdout).trim().to_string();
+        
+        if hash.is_empty() {
+            return Err("Empty git hash output".into());
+        }
+        
+        Ok(hash)
+    }
+
     /// Get currently active users
     fn get_active_users(&self) -> Result<Vec<String>, Box<dyn std::error::Error>> {
         let output = Command::new("who").output()?;
@@ -148,6 +171,31 @@ impl Collector for NixOSCollector {
             }
         }
 
+        // Collect config hash
+        match self.get_config_hash() {
+            Ok(hash) => {
+                metrics.push(Metric {
+                    name: "system_config_hash".to_string(),
+                    value: MetricValue::String(hash),
+                    unit: None,
+                    description: Some("NixOS configuration git hash".to_string()),
+                    status: Status::Ok,
+                    timestamp,
+                });
+            }
+            Err(e) => {
+                debug!("Failed to get config hash: {}", e);
+                metrics.push(Metric {
+                    name: "system_config_hash".to_string(),
+                    value: MetricValue::String("unknown".to_string()),
+                    unit: None,
+                    description: Some("Config hash (failed to detect)".to_string()),
+                    status: Status::Unknown,
+                    timestamp,
+                });
+            }
+        }
+
         // Collect agent hash
         match self.get_agent_hash() {
             Ok(hash) => {