Add exclude_email_metrics configuration option

- Add exclude_email_metrics field to AgentConfig for filtering email notifications
- Metrics matching excluded names skip notification processing but still appear in dashboard
- Optional field with serde(default) for backward compatibility
- Bump version to 0.1.56

agent/src/config/mod.rs

@@ -17,6 +17,9 @@ pub struct AgentConfig {
     pub notifications: NotificationConfig,
     pub status_aggregation: HostStatusConfig,
     pub collection_interval_seconds: u64,
+    /// List of metric names to exclude from email notifications
+    #[serde(default)]
+    pub exclude_email_metrics: Vec<String>,
 }
 
 /// ZMQ communication configuration