**What was implemented**  
I added the `langgraph` package to the project’s `requirements.txt` so that the import  
`from langgraph.graph import ...` works without raising a `ModuleNotFoundError`.

**Why this satisfies the requirement**  
The assignment explicitly asked to include `langgraph` in the dependencies. By listing it in `requirements.txt`, the package manager will install it during the environment setup, making the import statement valid.

**Key code excerpt**

```txt
# requirements.txt
langgraph
```

**Limitations**  
None – the change is minimal and directly addresses the reviewer’s feedback.