meta: add minimal AGENTS.md

[BethGriggs]

Now that the project’s AI use guidelines have landed (#62105), which permit the use of such tools, this proposes adding an AGENTS.md file to direct coding agents to the existing policies before they make changes.

Some tools may already discover and follow these documents, but this should make that behaviour slightly more deterministic. If it proves useful then we could consider extending with further instructions.

One consideration is that AGENTS.md is currently ignored (#62612). So, some contributors may have their own file which would subsequently conflict.

I know opinions vary, but the intent here is not to encourage usage of AI tools further than #62105, but to take the opportunity to guide the output.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.13%. Comparing base (21f0f27) to head (9e5a6ae).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65463      +/-   ##
==========================================
+ Coverage   90.12%   90.13%   +0.01%     
==========================================
  Files         752      751       -1     
  Lines      252315   252298      -17     
  Branches    47444    47437       -7     
==========================================
+ Hits       227395   227411      +16     
+ Misses      16217    16200      -17     
+ Partials     8703     8687      -16     

see 32 files with indirect coverage changes

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[bmuenzenmeyer]

I'm tempted to adopt the convention of suggesting a CLAUDE.md that only states @AGENTS.md per many community workarounds - but I like more that we support the open standard only instead of supporting proprietary formats.