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/mod.rs

@@ -119,7 +119,7 @@ impl TuiApp {
                 // Add NixOS metrics - using exact matching for build display fix
                 let nixos_metrics: Vec<&Metric> = all_metrics
                     .iter()
-                    .filter(|m| m.name == "system_nixos_build" || m.name == "system_active_users")
+                    .filter(|m| m.name == "system_nixos_build" || m.name == "system_active_users" || m.name == "system_agent_hash")
                     .copied()
                     .collect();
                 system_metrics.extend(nixos_metrics);