*codeplain client should fail if git is not installed

[dusano]

At the moment we get an ugly error if git is not installed on user's machine:

Upon running the client we should detect if git is installed and communicate to the user a nice error and exit.

[dusano]

This still doesn't work properly (see screenshot below). Moving this back to todo.

Steps to reproduce:

1. docker run -it --rm python:3.12-slim bash -c "apt-get update && apt-get install -y curl unzip && bash"
2. curl -fsSL https://codeplain.ai/install.sh | bash
3. codeplain --status