Add agent hash display to system panel

Implement agent version tracking to diagnose deployment issues:
- Add get_agent_hash() method to extract Nix store hash from executable path
- Collect system_agent_hash metric in NixOS collector
- Display "Agent Hash" in system panel under NixOS section
- Update metric filtering to include agent hash

This helps identify which version of the agent is actually running
when troubleshooting deployment or metric collection issues.

dashboard/src/ui/widgets/system.rs

@@ -16,6 +16,7 @@ pub struct SystemWidget {
     // NixOS information
     nixos_build: Option<String>,
     active_users: Option<String>,
+    agent_hash: Option<String>,
     
     // CPU metrics
     cpu_load_1min: Option<f32>,
@@ -64,6 +65,7 @@ impl SystemWidget {
         Self {
             nixos_build: None,
             active_users: None,
+            agent_hash: None,
             cpu_load_1min: None,
             cpu_load_5min: None,
             cpu_load_15min: None,
@@ -313,6 +315,11 @@ impl Widget for SystemWidget {
                         self.active_users = Some(users.clone());
                     }
                 }
+                "system_agent_hash" => {
+                    if let MetricValue::String(hash) = &metric.value {
+                        self.agent_hash = Some(hash.clone());
+                    }
+                }
                 
                 // CPU metrics
                 "cpu_load_1min" => {
@@ -397,6 +404,11 @@ impl Widget for SystemWidget {
             Span::styled(format!("Active users: {}", users_text), Typography::secondary())
         ]));
 
+        let agent_hash_text = self.agent_hash.as_deref().unwrap_or("unknown");
+        lines.push(Line::from(vec![
+            Span::styled(format!("Agent Hash: {}", agent_hash_text), Typography::secondary())
+        ]));
+
         // CPU section
         lines.push(Line::from(vec![
             Span::styled("CPU:", Typography::widget_title())