docs: strengthen workflow with mandatory checklist

AGENTS.md

@@ -23,11 +23,11 @@ This is a training project to learn functional programming in Python. Both Pytho
 - Challenge assumptions and ideas when it's useful. Be critical, not agreeable.
 
 ## Core Workflow: Research → Plan → Implement → Validate
-Start every feature with: "Let me research the codebase and create a plan before implementing."
+Before EVERY feature, complete this checklist:
+1. [ ] Research: Read relevant existing code
+2. [ ] Plan: Write to /.agent/current-plan.md and get user approval
+3. [ ] Implement: Code changes
+4. [ ] Validate: Run formatters, linters, and tests
 
-Research - Understand existing patterns and architecture
-Plan - Propose approach and verify with you
-Implement - Build with tests and error handling
-Validate - ALWAYS run formatters, linters, and tests after implementation
-Whenever working on a feature or issue, let's always come up with a plan first, then save it to a file called /.agent/current-plan.md, before getting started with code changes. Update this file as the work progresses.
+NEVER implement code before creating and getting approval on the plan file.
 Let's use pure functions where possible to improve readability and testing.