[Feature] Multi-LLM Provider Support

Extend LLM support beyond OpenAI to multiple 
  providers

  ## Target Providers
  - [ ] Claude (Anthropic)
  - [ ] Gemini (Google)
  - [ ] Local LLM (Ollama, LM Studio, etc.)

  ## Related Tasks
  - Add `--llm` CLI option
  - Extend `.env` configuration (ANTHROPIC_API_KEY, GEMINI_API_KEY, etc.)