Add section header for Agent Mode Note in README.md

Author: Raffaele Marcello

challenges/expensetracker/README.md

@@ -34,7 +34,7 @@ Build an **Expense Tracker** microservice that helps users log daily expenses an
 - Focus on current month data to keep calculations simple
 - Use mock data initially to test the UI
 
-### 
+### Agent Mode Note:
 If you are using the Agent mode and notice that the agent is working in the wrong directory, try to add the following line to your prompt:
 ```markdown
 Work in the directory backend. When run terminal remember to change the right directory, for example: `cd challenges/expensetracker/backend && <command>`.