Fix NixOS build version display in dashboard

Update metric filtering to use exact metric names instead of prefix matching.
This resolves the issue where build version showed 'unknown' despite agent
correctly collecting the metric.

dashboard/src/ui/mod.rs

@@ -119,7 +119,7 @@ impl TuiApp {
                 // Add NixOS metrics
                 let nixos_metrics: Vec<&Metric> = all_metrics
                     .iter()
-                    .filter(|m| m.name.starts_with("system_nixos") || m.name.starts_with("system_active"))
+                    .filter(|m| m.name == "system_nixos_build" || m.name == "system_active_users")
                     .copied()
                     .collect();
                 system_metrics.extend(nixos_metrics);

dashboard/src/ui/widgets/system.rs

@@ -297,6 +297,10 @@ impl Widget for SystemWidget {
         self.has_data = !metrics.is_empty();
 
         for metric in metrics {
+            // Debug logging for NixOS metrics
+            if metric.name.starts_with("system_") {
+                tracing::debug!("SystemWidget received metric: {} = {:?}", metric.name, metric.value);
+            }
             match metric.name.as_str() {
                 // NixOS metrics
                 "system_nixos_build" => {