[BUG] Fix inconsistency between documented and actual environment variable names for Gemini LLM

[adeeshperera]

The README documentation specifies using COMMIT_LLM=gemini and GEMINI_API_KEY environment variables, but the code was checking for COMMIT_LLM=google and GOOGLE_API_KEY, causing the tool to fail with "Invalid COMMIT_LLM value: gemini" when following the documented setup.

[DFanso]

hey @adeeshperera thank you for your issue can u work on this