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.

agent/src/collectors/nixos.rs

@@ -46,6 +46,23 @@ impl NixOSCollector {
         Ok(clean_version)
     }
 
+    /// Get agent hash from binary path
+    fn get_agent_hash(&self) -> Result<String, Box<dyn std::error::Error>> {
+        // Get the path of the current executable
+        let exe_path = std::env::current_exe()?;
+        let exe_str = exe_path.to_string_lossy();
+        
+        // Extract Nix store hash from path like /nix/store/fn804fh332mp8gz06qawminpj20xl25h-cm-dashboard-0.1.0/bin/cm-dashboard-agent
+        if let Some(store_path) = exe_str.strip_prefix("/nix/store/") {
+            if let Some(dash_pos) = store_path.find('-') {
+                return Ok(store_path[..dash_pos].to_string());
+            }
+        }
+        
+        // Fallback to "unknown" if not in Nix store
+        Ok("unknown".to_string())
+    }
+
     /// Get currently active users
     fn get_active_users(&self) -> Result<Vec<String>, Box<dyn std::error::Error>> {
         let output = Command::new("who").output()?;
@@ -131,6 +148,31 @@ impl Collector for NixOSCollector {
             }
         }
 
+        // Collect agent hash
+        match self.get_agent_hash() {
+            Ok(hash) => {
+                metrics.push(Metric {
+                    name: "system_agent_hash".to_string(),
+                    value: MetricValue::String(hash),
+                    unit: None,
+                    description: Some("Agent Nix store hash".to_string()),
+                    status: Status::Ok,
+                    timestamp,
+                });
+            }
+            Err(e) => {
+                debug!("Failed to get agent hash: {}", e);
+                metrics.push(Metric {
+                    name: "system_agent_hash".to_string(),
+                    value: MetricValue::String("unknown".to_string()),
+                    unit: None,
+                    description: Some("Agent hash (failed to detect)".to_string()),
+                    status: Status::Unknown,
+                    timestamp,
+                });
+            }
+        }
+
         debug!("Collected {} NixOS metrics", metrics.len());
         Ok(metrics)
     }