docs: add TDD red-orange-green workflow memo

AGENTS.md

@@ -25,6 +25,11 @@
    - Backend service existence is not enough.
    - The rendered page must actually consume the live data path.
 
+6. **Always work with TDD principles: Red, Orange, Green.**
+   - Start by making the failure visible with a test or reproducible check (Red).
+   - Iterate toward partial progress while keeping the failure scoped and observable (Orange).
+   - Finish only when the behavior is implemented and the test loop passes cleanly (Green).
+
 ## Project Learnings
 
 ### NiceGUI layout constraint