feat: Add image download and processing support

[konard]

Summary

This PR implements comprehensive image handling for pull requests, solving the "Could not process image" errors when used with Claude Code CLI. It adds support for downloading and validating embedded images from PR descriptions, comments, and reviews.

Key Features

• Image Download: Downloads all embedded images from PR body, comments, reviews, and review comments
• Multiple Formats: Supports both Markdown (![alt](url)) and HTML (<img src="url">) image references
• Image Validation: Validates downloaded files by checking magic bytes, not just file extensions. Detects:
  • PNG, JPG, GIF, WebP, BMP, ICO, SVG images
  • HTML error pages (GitHub 404 pages mistakenly downloaded)
• Redirect Handling: Properly follows GitHub's S3 signed URLs and redirects (up to 5 hops)
• Local Path Updates: Automatically updates markdown to reference local image paths
• JSON Output: New --format json option for programmatic use
• New CLI Options:
  • --download-images (default: true) - Download embedded images
  • --include-reviews (default: true) - Include PR reviews
  • --format markdown|json - Output format
  • --verbose / -v - Enable verbose logging

Output Structure

When saving to a directory:

pr-123.md               # PR content with local image references
pr-123-images/          # Directory with downloaded images
  image-1.png
  image-2.jpg
pr-123.json             # Optional JSON export (with --format json)

CLI Options

gh-download-pull-request <pr-url> [options]

Options:
  -t, --token           GitHub personal access token
  -o, --output          Output directory (default: current directory)
  --download-images     Download embedded images (default: true)
  --include-reviews     Include PR reviews (default: true)
  --format              Output format: markdown, json (default: markdown)
  -v, --verbose         Enable verbose logging
  -h, --help            Show help
  --version             Show version number

Usage Examples

# Download PR with images to current directory
gh-download-pull-request owner/repo#123 -o ./

# Output as JSON
gh-download-pull-request owner/repo#123 -o ./ --format json

# Skip image download
gh-download-pull-request owner/repo#123 --no-download-images

# Verbose mode for debugging
gh-download-pull-request owner/repo#123 -o ./ -v

Technical Implementation

1. Image Extraction: Uses regex to find both Markdown and HTML image references
2. Download with Auth: Uses GitHub token for authenticated image downloads
3. Magic Byte Validation: Checks file headers to identify actual format vs extension
4. Error Handling: Graceful handling of failed downloads, logs warnings but continues

Testing

• ✅ Tested with PRs containing images (badges, screenshots)
• ✅ All existing CLI tests pass
• ✅ Lint and format checks pass

Issue Reference

Fixes #7

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[konard]

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