Implement cross-host agent version comparison

- MetricStore tracks agent versions from all hosts
- Detects version mismatches using most common version as reference
- Dashboard logs warnings for hosts with outdated agents
- Foundation for visual version mismatch indicators in UI
- Helps identify deployment inconsistencies across infrastructure

dashboard/src/app.rs

@@ -236,6 +236,13 @@ impl Dashboard {
                     self.metric_store
                         .update_metrics(&metric_message.hostname, metric_message.metrics);
 
+                    // Check for agent version mismatches across hosts
+                    if let Some((current_version, outdated_hosts)) = self.metric_store.get_version_mismatches() {
+                        for outdated_host in &outdated_hosts {
+                            warn!("Host {} has outdated agent version (current: {})", outdated_host, current_version);
+                        }
+                    }
+
                     // Update TUI with new hosts and metrics (only if not headless)
                     if let Some(ref mut tui_app) = self.tui_app {
                         let mut connected_hosts = self

dashboard/src/metrics/store.rs

@@ -124,4 +124,52 @@ impl MetricStore {
             }
         }
     }
+
+    /// Get agent versions from all hosts for cross-host comparison
+    pub fn get_agent_versions(&self) -> HashMap<String, String> {
+        let mut versions = HashMap::new();
+        
+        for (hostname, metrics) in &self.current_metrics {
+            if let Some(version_metric) = metrics.get("agent_version") {
+                if let cm_dashboard_shared::MetricValue::String(version) = &version_metric.value {
+                    versions.insert(hostname.clone(), version.clone());
+                }
+            }
+        }
+        
+        versions
+    }
+
+    /// Check for agent version mismatches across hosts
+    pub fn get_version_mismatches(&self) -> Option<(String, Vec<String>)> {
+        let versions = self.get_agent_versions();
+        
+        if versions.len() < 2 {
+            return None; // Need at least 2 hosts to compare
+        }
+        
+        // Find the most common version (assume it's the "current" version)
+        let mut version_counts = HashMap::new();
+        for version in versions.values() {
+            *version_counts.entry(version.clone()).or_insert(0) += 1;
+        }
+        
+        let most_common_version = version_counts
+            .iter()
+            .max_by_key(|(_, count)| *count)
+            .map(|(version, _)| version.clone())?;
+        
+        // Find hosts with different versions
+        let outdated_hosts: Vec<String> = versions
+            .iter()
+            .filter(|(_, version)| *version != &most_common_version)
+            .map(|(hostname, _)| hostname.clone())
+            .collect();
+        
+        if outdated_hosts.is_empty() {
+            None
+        } else {
+            Some((most_common_version, outdated_hosts))
+        }
+    }
 }