feat: add standalone check-links command for external link validation

[Mearman]

Summary

Adds a new dedicated check-links command specifically optimized for external HTTP/HTTPS link validation, addressing the feature request in issue #37.

Key Features

🔄 Smart retry logic for temporary failures

• Configurable retry attempts (default: 3)
• Configurable delay between retries (default: 1000ms)
• Exponential backoff support

⚡ High-performance parallel checking

• Configurable concurrency (default: 10 concurrent requests)
• HEAD requests by default (faster than GET)
• Optional timeout configuration (default: 10s)

📊 Multiple output formats

• Text (human-readable console output)
• JSON (structured data for programmatic use)
• Markdown (formatted reports)
• CSV (spreadsheet-compatible)

🤖 Bot-detection handling

• Ignores 403/999 status codes by default (common bot-detection responses)
• Configurable ignore patterns with regex support
• Follow/no-follow redirects option

📈 Progress tracking and statistics

• Progress indicators for large documentation sets
• Response time measurement and averages
• Cache hit rate reporting (ready for future caching implementation)
• Domain-based analysis and grouping

Usage Examples

# Check all external links in current directory
markmv check-links

# Check with custom timeout and retry logic
markmv check-links docs/**/*.md --timeout 15000 --retry 5

# Generate JSON report
markmv check-links --format json > external-links-report.json

# Generate markdown report grouped by domain
markmv check-links --format markdown --group-by domain > report.md

# High concurrency checking with GET requests
markmv check-links --concurrency 20 --method GET --verbose

# Ignore specific patterns and status codes
markmv check-links --ignore-patterns "localhost,127.0.0.1" --ignore-status "403,999,503"

Implementation Details

• New command file: src/commands/check-links.ts
• CLI integration: Added to main CLI with comprehensive options
• Type-safe: Full TypeScript implementation with proper interfaces
• Error handling: Graceful handling of network failures and timeouts
• CI/CD friendly: Returns exit code 1 if broken links found

Comparison with existing validate --check-external

Feature	validate --check-external	check-links
Focus	General validation with external option	External links only
Retry logic	Basic	Smart retry with configurable delays
Output formats	Text, JSON	Text, JSON, Markdown, CSV
Progress tracking	Basic	Detailed with statistics
Bot handling	No	Built-in 403/999 ignore
Concurrency	Not configurable	Fully configurable
Domain grouping	No	Yes
Response times	No	Yes

Test Plan

• Command builds successfully
• Help text displays correctly
• Dry-run mode works as expected
• Basic CLI option parsing functions
• Test with actual external links (requires network)
• Test all output formats
• Test retry logic with failing links
• Test concurrency limits
• Test ignore patterns and status codes

Breaking Changes

None - this is a new command that doesn't affect existing functionality.

Related Issues

Resolves #37: Feature Request: Add standalone command for external link validation