Organize Agentbox runtime code

Co-authored-by: codegirl007 <codegirl-007@users.noreply.github.com>

agentbox/internal/agent/agent.go

@@ -14,6 +14,7 @@ type Observation struct {
 	PreviousActions []environment.InputAction
 }
 
+// Step is the compact history passed back to an agent on its next decision.
 type Step struct {
 	Number         int
 	Timestamp      time.Time
@@ -22,12 +23,14 @@ type Step struct {
 	Action         *environment.InputAction
 }
 
+// Decision contains either one action or Done. Returning neither is invalid.
 type Decision struct {
 	Reason string
 	Action *environment.InputAction
 	Done   bool
 }
 
+// Agent chooses one backend-neutral action from the latest observation.
 type Agent interface {
 	Name() string
 	NextAction(