Add repo-local CODEOWNERS enforcement

[tamirat-wubie]

Summary

• add repo-local .github/CODEOWNERS for enforceable ownership routing
• reference only closed/visible Mullusi teams with repository permissions already granted

Changed files

• .github/CODEOWNERS

Verifier command

powershell gh api repos/mullusi/mullusi-docs/contents/.github/CODEOWNERS?ref=codex/codeowners-enforcement-20260525 --jq .path

Validation result

• Readback passed: $readback
• Branch created from main
• Protected main was not bypassed

Rollback path

• Close this PR before merge, or revert the merge commit after merge.

[tamirat-wubie]

Validation result: validate passed. Readback verifier passed in PR body. Rollback: close PR or revert merge commit.