Add JSON output and headless mode for dashboard integration

[jeremyfelt]

Summary

Add machine-readable output formats and headless operation mode to support dashboard integration and CI/CD pipelines.

Use Cases

• Dashboard showing visual regression status across projects
• CI/CD pipeline integration with pass/fail results
• Automated reporting and notifications
• Integration with external monitoring systems

Tasks

• Add --json flag for machine-readable output
• Implement --silent/--headless mode (no browser opening)
• Add proper exit codes (0 = no differences, 1 = differences found, 2 = error)
• Create structured output format for comparison results
• Support webhook notifications for changes
• Add summary statistics (total comparisons, differences found, etc.)

JSON Output Format

{
  "timestamp": "2024-01-01T12:00:00Z",
  "property": "homepage",
  "status": "differences_found",
  "summary": {
    "total_comparisons": 6,
    "differences_found": 2,
    "max_difference_percentage": 1.5
  },
  "results": [
    {
      "url": "http://localhost:3000",
      "viewport": "desktop",
      "difference_percentage": 1.5,
      "html_changes": true,
      "diff_image_path": "./compares/homepage-home-desktop-diff.png"
    }
  ]
}

Exit Codes

• 0: Success, no visual differences
• 1: Success, but visual differences found
• 2: Error (missing files, invalid config, etc.)

CLI Flags

• --json: Output results in JSON format
• --silent: Don't open browser reports
• --webhook: POST results to webhook URL
• --threshold: Override difference threshold for pass/fail

Acceptance Criteria

• JSON output contains all necessary information for dashboards
• Exit codes properly indicate success/failure states
• Silent mode works without opening browsers
• Webhook integration functions correctly
• Backward compatibility maintained for interactive usage