Add debug logging to diagnose Phase 4 service discovery issue

Add detailed debug logging to track:
- Service discovery start
- Individual service parsing
- Final service count and list
- Empty results indication

This will help identify why cmbox disappeared from dashboard.

agent/src/collectors/systemd.rs

@@ -127,6 +127,7 @@ impl SystemdCollector {
 
     /// Auto-discover interesting services to monitor
     fn discover_services(&self) -> Result<Vec<String>> {
+        debug!("Starting systemd service discovery with status caching");
         // Get all services (includes inactive, running, failed - everything)
         let units_output = Command::new("systemctl")
             .arg("list-units")
@@ -175,6 +176,7 @@ impl SystemdCollector {
                 });
 
                 all_service_names.insert(service_name.to_string());
+                debug!("Parsed service: {} (load:{}, active:{}, sub:{})", service_name, load_state, active_state, sub_state);
             }
         }
 
@@ -219,6 +221,11 @@ impl SystemdCollector {
             state.service_status_cache = status_cache;
         }
 
+        debug!("Service discovery completed: found {} matching services: {:?}", services.len(), services);
+        if services.is_empty() {
+            debug!("No services found matching the configured filters - this may indicate a parsing issue");
+        }
+
         Ok(services)
     }