Add AGENTS.md distilled from the recent PRs

[ViralBShah]

Adds a root AGENTS.md for agents and other contributors, distilled from the pull requests merged over the last three years and the current test runner, CI and wrapper generator. It states general rules rather than the history behind them.

Every function, macro and helper the file names was checked against the current source.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KQYs2iu5TJLehhvpSCRHvV

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.42%. Comparing base (f4b0d50) to head (eaf8719).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #826      +/-   ##
==========================================
+ Coverage   92.36%   92.42%   +0.05%     
==========================================
  Files          12       12              
  Lines        8632     8682      +50     
==========================================
+ Hits         7973     8024      +51     
+ Misses        659      658       -1     

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