doc(AGENTS.md): guidelines for AI assisted contrib

[bassosimone]

We have seen a flurry of contributions recently, some of which look like assisted by AI. I am authoring these changes to try to mitigate the effort required by me or other maintainers to go through the changes. These changes encoded what I just learned while going through some contributions. I will continue to iterate on making these instructions better.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (2c6a185) to head (33c4c0f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #159   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          52       52           
  Lines        3729     3729           
=======================================
  Hits         3726     3726           
  Misses          3        3           

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