feat: add experiment log for phase transition

ref: N25B-453

src/control_backend/agents/user_interrupt/user_interrupt_agent.py

@@ -197,6 +197,7 @@ class UserInterruptAgent(BaseAgent):
             case "transition_phase":
                 new_phase_id = msg.body
                 self.logger.info(f"Phase transition detected: {new_phase_id}")
+                experiment_logger.observation("Transitioned to next phase.")
 
                 payload = {"type": "phase_update", "id": new_phase_id}