AGENTS.md first draft

[jlnav]

Guided by reading through https://agents.md/ and many of the examples.

Concentrated information about libEnsemble and considerations for developing with/for it, for LLMs.

Asked Gemini and Claude for feedback several times.

https://github.com/Libensemble/libensemble/pull/1687/changes#diff-a54ff182c7e8acf56acfd6e4b9c3ff41e2c41a31c9b211b2deb9df75d9a478f9

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.39%. Comparing base (afe97f9) to head (5bf37f1).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1687   +/-   ##
========================================
  Coverage    78.39%   78.39%           
========================================
  Files           81       81           
  Lines         8257     8257           
  Branches      1266     1266           
========================================
  Hits          6473     6473           
  Misses        1548     1548           
  Partials       236      236           

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