Add final_answer prompt template

prompts.yaml

@@ -180,6 +180,12 @@
   10. Don't give up! You're in charge of solving the task, not providing directions to solve it.
 
   Now Begin! If you solve the task correctly, you will receive a reward of $1,000,000.
+
+"final_answer": |-
+  You have reached the final step of your task. Use the final_answer tool to submit your answer.
+  Make sure your answer is clear, concise, and directly addresses the original question.
+  If you're unsure about any part of your answer, explain your reasoning.
+
 "planning":
   "initial_facts": |-
     Below I will present you a task.