chore: reference sources to react, react-native, testing-libraries, etc

[mdjastrzebski]

Summary

Add React, React Native, DTL, RTL, and Expensify/App as shallow git submodules under refs/, for agents to read upstream code and real-world usage. Jest, ESLint, oxfmt, and VS Code search ignore refs/. AGENTS.md documents each ref and how to use and refresh them.

Test plan

• yarn jest --listTests: no tests from refs/.
• yarn oxfmt --check on changed docs: passes.
• yarn validate not run yet.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.56%. Comparing base (821ef64) to head (f506437).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1948   +/-   ##
=======================================
  Coverage   98.56%   98.56%           
=======================================
  Files          87       87           
  Lines        1744     1744           
  Branches      459      459           
=======================================
  Hits         1719     1719           
  Misses         24       24           
  Partials        1        1           

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