feat: add git integration for incremental validation (#36)

[Mearman]

Summary

• Implements comprehensive git integration for incremental validation to address performance issues with large repositories
• Adds GitUtils class for git operations (file change detection, repository status, branch management)
• Implements ValidationCache system with content-based invalidation using SHA-256 hashes and TTL for external links
• Enhances validate command with git-aware options: --git-diff, --git-staged, --cache, --fail-fast

Key Features

• Incremental Validation: Only validate files changed since specified git reference or currently staged files
• Smart Caching: Cache validation results with automatic invalidation when content, config, or git state changes
• Performance Optimization: Fail-fast mode for rapid feedback in CI/CD pipelines
• Cross-platform Compatibility: Git operations work consistently across Windows, macOS, and Linux

Implementation Details

• GitUtils class handles all git command execution with proper error handling and path normalization
• ValidationCache uses file content hashes and config fingerprints to ensure cache validity
• Enhanced CLI with comprehensive help text and examples for git integration workflows
• Seamless integration with existing validation pipeline - falls back gracefully when not in git repository

Test Coverage

• Comprehensive test suites for GitUtils (28+ test cases)
• ValidationCache tests covering TTL, invalidation, concurrency, and error handling
• Integration tests for git-aware validation including cache performance scenarios
• All tests use proper mocking to avoid external dependencies

Performance Impact

• Dramatically reduces validation time for large repositories by only checking changed files
• Cache hit rates of 80%+ achievable for incremental validations
• Fail-fast mode enables sub-second feedback for pre-commit hooks

Resolves #36