feat: add AGENTS.md for Claude Code etc.

[bradenmacdonald]

Description

This adds an AGENTS.md file that Claude Code and other AI coding tools will read, to help save time by ensuring that the agents are aware of our current best practices for this repo.

Impacts: developers

Supporting information

Discussed on Slack: https://openedx.slack.com/archives/C04BM6YC7A6/p1774912475830039

Private ref: MNG-4670

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.69%. Comparing base (8d5842b) to head (b921343).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3120      +/-   ##
==========================================
+ Coverage   95.66%   95.69%   +0.03%     
==========================================
  Files        1402     1400       -2     
  Lines       33224    33372     +148     
  Branches     7529     7809     +280     
==========================================
+ Hits        31783    31937     +154     
+ Misses       1394     1373      -21     
- Partials       47       62      +15     

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