Lab: The agent loop moves into the package

Module 5 · Lab · About 75 minutes · Needs: the Module 3 lab's tools.py, and lessons 1 and 3 (run_agent, over_limit, act and build_graph; interrupt, the SQLite checkpointer and RunControl); Lesson 2 for the reasons a loop is worth its cost

What you are building

This lab moves the module's loop into returns_desk/loop.py, so later modules import it instead of pasting it. The file holds Lesson 1's loop with its four limits and named stop reasons, the same loop as a LangGraph graph, and Lesson 3's approval graph: it pauses before Aisha's $129.00 refund, survives a restart, and stops when Priya cancels. Every tool call still goes through Module 3's call, imported from tools.py, never copied. Tests pin the stop reasons with a stand-in model, then a real model works two requests. Everything at BrightCart is simulated.