feat: add intelligent AI context generator for enhanced code reviews

Add comprehensive context generation system that automatically analyzes projects and creates detailed context files to improve AI code review quality.

• Add ai-generate-context CLI tool with multi-provider support (Anthropic, Gemini, Ollama)
• Implement automatic project analysis: dependencies, structure, code patterns
• Create modular architecture with facts extraction, LLM analysis, and template rendering
• Add section-based system with automatic and manual content preservation
• Support partial updates (--section overview, tech_stack, structure, review_focus)
• Include comprehensive test coverage with unit and integration tests
• Add complete documentation and user guides

Key features:

• Hybrid analysis combining deterministic facts with LLM-driven insights
• Template engine with manual section preservation across updates
• Seamless integration with ai-code-review workflow via .ai_review/project.md
• Generates project overview, technology stack, architecture patterns, and review focus areas

The context generator enhances AI code reviews by providing rich project context, leading to more accurate and relevant feedback.