Add contribution guidelines, including an AI use policy

[dralley]

And an AGENTS.md / CLAUDE.md, so that it gets picked up by tooling.

[dralley]

Claude, for whatever reason is the only harness which doesn't support AGENTS.md. Annoying.

This is a symlink

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.95%. Comparing base (e00ae5c) to head (9c09acf).
⚠️ Report is 8 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #985      +/-   ##
==========================================
- Coverage   57.31%   56.95%   -0.36%     
==========================================
  Files          46       47       +1     
  Lines       18197    18508     +311     
==========================================
+ Hits        10429    10542     +113     
- Misses       7768     7966     +198     

Flag	Coverage Δ
unittests	56.95% <ø> (-0.36%)	⬇️

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