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/agent.go

@@ -80,6 +80,12 @@ func New(tm *manager.TraderManager, st *store.Store, cfg *Config, logger *slog.L
 
 func (a *Agent) SetAIClient(c mcp.AIClient) { a.aiClient = c }
 
+func (a *Agent) ensureHistory() {
+	if a.history == nil {
+		a.history = newChatHistory(100)
+	}
+}
+
 func (a *Agent) log() *slog.Logger {
 	if a != nil && a.logger != nil {
 		return a.logger