ci: rename secret to LITELLM_API_KEY in reusable review workflow

[AtMrun]

Summary

Rename AI_GATEWAY_KEY to LITELLM_API_KEY in the reusable Claude review workflow to match the application-sdk convention. This way connector repos that already have LITELLM_API_KEY configured don't need a new secret.

🤖 Generated with Claude Code

[snykgituser]

✅ Snyk checks have passed. No issues have been found so far.

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[Aryamanz29]

Closing — the reusable review workflow this PR targets has been replaced/removed on main. The secret naming convention has evolved since. Branch preserved.