Add agent version reporting and display

- Agent reports version via agent_version metric using nix store hash
- Dashboard displays agent version in system widget
- Foundation for cross-host version comparison
- Both agent -V and dashboard show versions

dashboard/src/ui/widgets/system.rs

@@ -339,9 +339,9 @@ 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());
+                "agent_version" => {
+                    if let MetricValue::String(version) = &metric.value {
+                        self.agent_hash = Some(version.clone());
                     }
                 }