fix(AGENTS.md): make AI instructions stricter

[bassosimone]

This diff makes instructions even more strict. We just discussed these changes with @robertodauria.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (91f1fc0) to head (ffb5feb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #161   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
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  Files          52       52           
  Lines        3729     3729           
=======================================
  Hits         3726     3726           
  Misses          3        3           

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