Change docker image display format and status

Changes:
- Rename docker images from 'image_node:18...' to 'I node:18...' for conciseness
- Change image status from 'active' to 'inactive' for neutral informational display
- Images now show with gray empty circle ○ instead of green filled circle ●

Docker images are static artifacts without meaningful operational status, so using inactive status provides neutral gray display that won't trigger alerts or affect service status aggregation.

agent/src/collectors/systemd.rs

@@ -865,8 +865,8 @@ impl SystemdCollector {
                 let size_mb = self.parse_docker_size(size_str);
 
                 images.push((
-                    format!("image_{}", image_name),
-                    "active".to_string(), // Images are always "active" (present)
+                    format!("I {}", image_name),
+                    "inactive".to_string(), // Images are informational - use inactive for neutral display
                     size_str.to_string(),
                     size_mb
                 ));