chore: update markdown files disallowing internal default params

[scotthart]

Update and/or emphasize that internal functions should not have default parameters.

[gemini-code-assist]

Code Review

This pull request updates the .agents/AGENTS.md file to add a new guideline discouraging the use of default arguments in internal namespaces. It explains that default parameters obscure dependencies and can mask untested branches in unit tests, recommending explicit overloads instead. There are no review comments to address, and I have no additional feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.25%. Comparing base (30817a8) to head (a51b8f1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16373      +/-   ##
==========================================
- Coverage   92.26%   92.25%   -0.01%     
==========================================
  Files        2239     2239              
  Lines      210719   210719              
==========================================
- Hits       194412   194406       -6     
- Misses      16307    16313       +6     

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