feat: Add comprehensive web clipper functionality for issue #38

[Mearman]

Summary

Implements comprehensive web page to markdown conversion functionality as requested in issue #38. This adds a powerful clip command to the markmv CLI that can extract content from various types of web pages using multiple intelligent strategies.

Key Features

🚀 Multi-Strategy Content Extraction

• Auto Strategy: Intelligently selects the best extraction method based on content patterns
• Readability Strategy: Uses Mozilla Readability algorithm for clean article extraction
• Manual Strategy: Custom CSS selectors for complex or non-standard sites
• Full Strategy: Complete page extraction including navigation and footers
• Structured Strategy: Schema.org and JSON-LD structured data extraction

🌐 Comprehensive Site Support

• Blog Posts & Articles: Optimized extraction with Readability algorithm
• Documentation Sites: Custom selector support for technical docs
• Single Page Applications: Multiple strategies to handle dynamic content
• E-commerce & Complex Sites: Flexible extraction options
• News Sites & Publications: Metadata-aware extraction with author/date

📋 Advanced Processing Options

• Image Handling: Skip, link-only, download locally, or embed as base64
• Link Classification: Automatic internal/external link detection and processing
• Metadata Extraction: Title, author, published date, description with fallbacks
• Frontmatter Generation: YAML frontmatter with configurable metadata inclusion

🔧 Professional Features

• Authentication Support: Custom headers, cookies, and user-agent strings
• Batch Processing: Process multiple URLs from text files
• Error Handling: Comprehensive error recovery and detailed reporting
• Dry-Run Mode: Preview extraction without creating files
• JSON Output: Programmatic interface for integration with other tools

Implementation Highlights

🏗️ Architecture

• Core WebClipper Class: Main extraction engine with strategy pattern
• CLI Command Interface: Full-featured command with extensive options
• TypeScript Strict Mode: Zero type coercion, comprehensive interfaces
• Modular Design: Extensible strategy system for future enhancements

🧪 Testing

• 42 Total Tests: Comprehensive coverage of all functionality
• Core Tests (28): WebClipper class methods, strategies, error handling
• CLI Tests (14): Command interface, option parsing, file operations
• Mock Integration: External dependencies properly mocked (jsdom, readability, turndown)

🔒 Quality Assurance

• Strict TypeScript: No any types in production code
• Error Boundaries: Graceful handling of network issues, parsing failures
• Cross-Platform: Proper filename sanitization and path handling
• Resource Management: Timeout controls and memory-conscious processing

Usage Examples

# Basic article extraction
markmv clip https://example.com/article

# Custom output location
markmv clip https://blog.com/post -o article.md

# Batch processing with custom strategy
markmv clip urls.txt --batch --strategy manual --selectors "article,.content"

# Download images locally
markmv clip https://tutorial.com --image-strategy download --image-dir ./images

# Authentication-aware extraction
markmv clip https://protected.com --headers '{"Authorization": "Bearer token"}'

# Dry-run with verbose output
markmv clip https://example.com --dry-run --verbose

CLI Integration

The new clip command integrates seamlessly with the existing markmv CLI:

• Follows established patterns for options and output formatting
• Supports dry-run mode consistent with other commands
• Provides JSON output for programmatic usage
• Includes comprehensive help text and examples

Test Coverage

All functionality is thoroughly tested:

• ✅ Strategy detection and selection
• ✅ Content extraction from various HTML structures
• ✅ Metadata parsing and frontmatter generation
• ✅ Image and link processing
• ✅ Error handling and recovery
• ✅ CLI option parsing and validation
• ✅ Batch processing workflows
• ✅ File output and directory creation

Technical Implementation

• Dependencies: Added @mozilla/readability, turndown, jsdom, node-html-parser
• Type Safety: Comprehensive interfaces without any type coercion
• Performance: Efficient parsing with timeout controls and resource management
• Maintainability: Clean separation of concerns with strategy pattern
• Extensibility: Easy to add new extraction strategies or output formats

Closes #38

Test Plan

• All existing tests pass
• New functionality has comprehensive test coverage
• CLI integration works correctly
• Error handling is robust
• Cross-platform compatibility verified
• TypeScript strict mode compliance
• Linting standards met (test file warnings acceptable)