Add an AGENTS.md file to help with AI-assisted debugging/development

In this time and age, AI is everywhere. However, it's sometimes not very
easy to use. For Greenfield projects it works quite a bit better than
for existing legacy projects. And Git's source code is _quite_ as legacy
code as they come... 😁

Now, the only way how AI can be used efficiently with legacy code
is by providing enough information by way of prompt context for the
AI to have a chance to make any sense of the code. The structure and
the architecture is, after all, not designed for AI, but rather the
opposite: By virtue of having grown organically over two decades, there
is no design that AI coding models would readily grasp.

So here is a document that describes all kinds of aspects about this
project. The idea is to help AI by providing information that it does
not have ingrained in its weights. The idea is to provide information
that a human prompter might take for granted, but no coding model will
have been trained on specifically.

Assisted-by: Claude Opus 4.5
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho