[Feature] Add OpenAI GPT as LLM Provider

[DFanso]

Add support for OpenAI's GPT models as an additional LLM provider option.

Requirements:

• Add new provider option in COMMIT_LLM environment variable
• Create src/openai/openai.go with GenerateCommitMessage function
• Follow the same pattern as gemini.go and grok.go
• Update README.md with OpenAI setup instructions

Resources:

• OpenAI API Docs: https://platform.openai.com/docs/api-reference