fix(agent): add TargetRef nil guards and ensureHistory for robustness

- Add nil checks for session.TargetRef in all four execute*Action
  handlers (Trader/Exchange/Model/Strategy) to prevent panic on
  corrupted sessions; bulk-delete and query actions are excluded
- Add ensureHistory() helper and call it in runPlannedAgentWithContextMode
  to prevent nil panic when history is not initialized

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

agent/planner_runtime.go

@@ -2551,6 +2551,7 @@ func (a *Agent) runPlannedAgentWithContextMode(ctx context.Context, storeUserID
 		answer, _, err := a.driveActiveSession(ctx, storeUserID, userID, lang, text, session, onEvent)
 		return answer, err
 	}
+	a.ensureHistory()
 	a.history.Add(userID, "user", text)
 	if onEvent != nil {
 		onEvent(StreamEventPlanning, a.planningStatusText(lang))