AGENTS.md: adapt it to the Microsoft Git fork

The Microsoft Git fork is a downstream fork of Git for Windows. Its
reason to exist is quite a bit different from Git for Windows. For
example, it targets not only Windows, it also supports Linux and macOS
builds. Historically, its main goal has been to support VFSforGit. In
the meantime, also Scalar targeting Azure Repos has been a major focus
of this repository. These aspects need to be reflected in the agents. So
that LLM assistance can be more effective.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho