meta: ignore AI assistants files

[mcollina]

• Add CLAUDE.md and AGENTS.md to .gitignore under a new AI assistants section
• Exclude them from markdown linting in Makefile via $(filter-out ...)
• Exclude them from ESLint markdown processing in eslint.config.mjs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.79%. Comparing base (449a93a) to head (2fb4df5).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #62612   +/-   ##
=======================================
  Coverage   89.78%   89.79%           
=======================================
  Files         697      697           
  Lines      215749   215749           
  Branches    41294    41293    -1     
=======================================
+ Hits       193705   193726   +21     
+ Misses      14124    14118    -6     
+ Partials     7920     7905   -15     

see 33 files with indirect coverage changes

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