Provide context on how to correctly call butler in AGENTS.md (#5286)

## Overview
This PR updates `AGENTS.md` to provide clear instructions on how to
correctly use `butler.py` with `pipenv`, ensuring agents operate within
the virtual environment.

Currently, agents always fail when calling butler commands because they
don't know(yet) that butler commands fail if out of a pipenv
environment, hence you are always correcting it, or waiting for it till
it reruns commands in `pipenv` .

Author: IvanBM18

AGENTS.md

@@ -54,6 +54,20 @@ Now you are ready to install the project's dependencies. Run the following comma
 ./local/install_deps.bash
 ```
 
+## Using Butler
+Before each time you use the `butler.py` script, verify that you are inside the virtual environment, if not activate it by running `python -m pipenv shell`, or pre append any `python butler.py` call with
+`pipenv run`
+
+For instance:
+```bash
+pipenv run python butler.py lint
+```
+Is the same as running
+```bash
+python butler.py lint
+```
+if you are inside a virtual environment.
+
 ## Testing
 
 To run all unit tests, execute the following commands: