Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
Trojan.Agent.Trace is the generic detection for traces of infection that can’t be attributed by a specific Trojan.
Trojan.Agent.Trace is harmless by itself, but the presence is a sign of system infection. The traces could be leftovers from an already removed malware.
Protection against Trojan.Agent.Trace is not necessary. This is merely a removal detection
Malwarebytes can detect and remove Trojan.Agent.Trace without further user interaction.