Feature/add agents md

[pnoltes]

This pull request add a AGENTS.md and a Using Generative AI paragraph to the Apache Celix code conventions.

I am not sure if the AGENTS.md will become a standard, but I think it good to start describing how coding agents should work with the Apache Celix project; And for now I think a AGENTS.md is a clean choice (not bound to a GenAI vendor).

I also think it is important to clearly state that using Generative AI does transfer the responsibility of writing clean and working code.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.49%. Comparing base (c6d0e2a) to head (986eb9b).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #814      +/-   ##
==========================================
+ Coverage   91.44%   91.49%   +0.04%     
==========================================
  Files         234      234              
  Lines       28655    28655              
==========================================
+ Hits        26204    26217      +13     
+ Misses       2451     2438      -13     

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[PengZheng]

LGTM