Added

• Thumbnail cache for efficient image serving via HTTP URLs (/thumbnail/:cache_key)
• New tool pcli2_thumbnail_cache_cleanup to remove expired thumbnails
• Disk-based cache at ~/.pcli2-mcp/thumbnails/ with configurable TTL (default 24 hours)
• HTML response for pcli2_asset_thumbnail with embedded image URL instead of base64 data

Changed

• pcli2_asset_thumbnail now returns HTML with image URL instead of base64-encoded image data
• AppState extended to include optional thumbnail cache reference