Phase 1: Implement exact name filtering for service matching

Change service matching logic from contains-based to exact equality.
Services now match only if service_name == pattern exactly.
This is the first step in the systemd collector optimization plan.

agent/src/collectors/systemd.rs

@@ -253,7 +253,7 @@ impl SystemdCollector {
 
                 // Check if this service matches our filter patterns
                 for pattern in service_name_filters {
-                    if service_name.contains(pattern) || pattern.contains(service_name) {
+                    if service_name == pattern {
                         debug!(
                             "INCLUDING service '{}' because it matches pattern '{}'",
                             service_name, pattern