Fix service disk usage detection to use sudo du for permission access

ARK service directories require elevated permissions to access. The NixOS
configuration already allows sudo du with NOPASSWD, so use sudo du instead
of direct du command to properly detect disk usage for restricted directories.

agent/src/collectors/systemd.rs

@@ -294,7 +294,7 @@ impl SystemdCollector {
 
     /// Get directory size in GB with permission-aware logging
     fn get_directory_size(&self, dir: &str) -> Option<f32> {
-        let output = Command::new("du").arg("-sb").arg(dir).output().ok()?;
+        let output = Command::new("sudo").arg("du").arg("-sb").arg(dir).output().ok()?;
 
         if !output.status.success() {
             // Log permission errors for debugging but don't spam logs