Add initial AGENTS.md for guiding AI agents

[djhoese]

This adds an initial version of AGENTS.md to guide AI agents in working with pyresample. Most of this was generated with Claude, but since it was heavily influenced by my "concepts" documentation it is still pretty human-ish in my opinion.

This PR includes a couple fixes that I noticed while reviewing what Claude was talking about.

• Closes #xxxx
• Tests added
• Tests passed
• Fully documented

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.71%. Comparing base (8aca19c) to head (71f7a5b).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #744      +/-   ##
==========================================
+ Coverage   93.68%   93.71%   +0.03%     
==========================================
  Files          89       89              
  Lines       13721    13716       -5     
==========================================
  Hits        12854    12854              
+ Misses        867      862       -5     

Flag	Coverage Δ
unittests	93.71% <ø> (+0.03%)	⬆️

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[djhoese]

Merging this now. I can always make updates later.