Add CODEOWNERS in both root and .github locations

[jasmeralia]

Summary

• Adds a CODEOWNERS file in both the repo root and .github/ so the review-request mechanism works regardless of which location a tool checks first.
• On private repos that previously had no CODEOWNERS at all (steam-typhoon, mfc-chat-overlay), this also adds the file for the first time, along with removing the dead reviewers key from dependabot.yml (private repos on this account currently don't get CODEOWNERS-based auto-review-request without GitHub Pro, but this is harmless to add now and will start working automatically if that changes).

Test plan

• Verified both CODEOWNERS files contain identical content (* @jasmeralia)
• Verified dependabot.yml has no remaining reviewers key

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.38%. Comparing base (d8b476b) to head (59f3a4d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #23   +/-   ##
=======================================
  Coverage   82.38%   82.38%           
=======================================
  Files          39       39           
  Lines        8082     8082           
=======================================
  Hits         6658     6658           
  Misses       1424     1424           

Flag	Coverage Δ
unittests	82.38% <ø> (ø)

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