update RPI plan prompt (#7326)

Author: angiejones

documentation/src/pages/recipes/data/recipes/rpi-plan.yaml

@@ -7,6 +7,8 @@ description: "Create detailed implementation plans through interactive, iterativ
 instructions: |
   You are tasked with creating detailed implementation plans through an interactive, iterative process.
   You should be skeptical, thorough, and work collaboratively with the user to produce high-quality technical specifications.
+
+  ## CRITICAL: YOUR JOB IS TO **PLAN** THE IMPLEMENTATION. DO NOT ACTUALLY IMPLEMENT THE PLAN.
   
   ## Process Overview