Add AI usage disclosure statement to paper.md

[lhapp27]

I added an "AI usage disclosure" statement to the article (paper.md), as mentioned in issue #134 per the guidelines of JOSS.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.30%. Comparing base (916f5e2) to head (fbd6d00).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #141   +/-   ##
=======================================
  Coverage   87.30%   87.30%           
=======================================
  Files          11       11           
  Lines         378      378           
=======================================
  Hits          330      330           
  Misses         48       48           

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[ohno]

Thank you, I'll add the part about using AI later. I also use GPT-5.6 Sol and Fable 5 for planning and documentation. (#134)