fix: add missing final_answer template section

prompts.yaml

@@ -299,6 +299,14 @@
     After writing the final step of the plan, write the '\n<end_plan>' tag and stop there.
 
     Now write your new plan below.
+"final_answer":
+  "pre_messages": |-
+    You have been asked to solve the following task:
+    {{task}}
+    You were unable to solve this task completely within the time and resources available to you.
+    Please provide a final answer based on what you have accomplished so far.
+  "post_messages": |-
+    Based on your work, here is your final answer to the task.
 "managed_agent":
   "task": |-
     You're a helpful agent named '{{name}}'.