Skip LastAgent in Result JSON serialization

Result.LastAgent is *Agent, which holds only unexported fields. The
default json.Marshal renders it as an empty object, which is misleading
when persisted alongside the agent run.

Tag LastAgent as json:"-" and add explicit lowercase JSON tags to the
remaining fields so the serialized shape is stable for callers that
read the persisted result column.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/agent/result.go

@@ -21,7 +21,7 @@ type (
 		Messages  []llm.Message
 		Usage     llm.Usage
 		Turns     int
-		LastAgent *Agent
+		LastAgent *Agent `json:"-"`
 	}
 )