feat: Add heading refactoring command with comprehensive link updates

[Mearman]

Summary

Implements a comprehensive heading refactoring command that allows users to update heading text across multiple markdown files while automatically maintaining link integrity.

Features

• Heading Text Updates: Updates all instances of specified heading text across files
• Automatic Slug Generation: Generates URL-friendly anchor slugs using consistent algorithm
• Anchor Link Updates: Automatically updates all anchor links that reference changed headings
• Cross-File References: Updates heading references across the entire project
• Custom Slug Generation: Support for custom slugify functions for specialized use cases
• Recursive Processing: Process directories recursively with depth control
• Dry Run Support: Preview changes before applying them
• Comprehensive Reporting: Detailed reporting of all heading changes and link updates

Implementation Details

• Added src/commands/refactor-headings.ts with full functionality
• Leverages existing TocGenerator for consistent slug generation
• Uses LinkParser for anchor link detection and updates
• Supports glob patterns and directory processing
• Comprehensive error handling with graceful degradation

Testing

• Added 15 comprehensive test cases covering all functionality
• Integration tests with real filesystem operations
• Tests for heading updates, link updates, dry-run mode, error handling
• Custom slug generation and regex escaping edge cases
• Multi-file processing and cross-reference scenarios

Usage Examples

# Basic heading refactoring
markmv refactor-headings docs/ --old-heading "Getting Started" --new-heading "Quick Start Guide" --recursive

# With custom options
markmv refactor-headings README.md --old-heading "Installation" --new-heading "Setup" --dry-run

# Skip cross-references
markmv refactor-headings **/*.md --old-heading "API" --new-heading "API Reference" --no-update-cross-references

Test Coverage

All tests pass with comprehensive coverage:

• ✅ Basic heading refactoring with duplicate headings
• ✅ Multiple heading levels and exact text matching
• ✅ Anchor link updates with proper slug mapping
• ✅ Cross-reference update control
• ✅ Dry-run mode functionality
• ✅ Multi-file processing
• ✅ Error handling for missing files
• ✅ Custom slug generation support
• ✅ Special character handling in headings
• ✅ Comprehensive result formatting

Closes #31