Integrates `gptel-magit` to provide AI assistance directly within
Magit. Disables `gptel-include-reasoning` as a temporary workaround
for an upstream issue (ragnard/gptel-magit#8).
I reordered the code in a more clean an organized way. Also this improves a lot
the emacs startup time again, bucause after some updates it becomes a bit slow
with the old configuration, so i did it again in a new way.
