🎉 MAJOR UPDATE: Multi-LLM Provider Support

Features:
- ✨ Support for 5 LLM providers (Gemini, OpenAI, Claude, Mistral, Groq)
- 🔧 Enhanced interactive setup wizard with provider selection
- 📊 Flexible configuration (CLI flags, config file, env vars)
- ⚡ Optional dependencies per provider
- 🎯 Provider-specific model selection

New Files:
- src/codesonor/llm_providers.py - Provider abstraction layer
- MULTI_LLM.md - Comprehensive multi-LLM documentation

Updated:
- ai_analyzer.py - Multi-provider support
- analyzer.py - LLM provider parameters
- config.py - Multi-provider configuration management
- cli.py - New LLM options (--llm-provider, --llm-api-key, --llm-model)
- README.md - Updated with multi-LLM features
- CLI_README.md - Provider-specific docs
- tests/test_codesonor.py - Multi-provider tests

Breaking Changes: None (fully backwards compatible)

Migration: Old gemini_key configs automatically converted