feat: add flat-doodle style to cover-image and article-illustrator

Add new flat-doodle style featuring:
- Bold black outlines
- Bright pastel colors (pink, mint, lavender, yellow)
- Simple flat shapes with cute rounded proportions
- White background
- Best for productivity, SaaS, workflow content

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

skills/baoyu-cover-image/references/styles/flat-doodle.md

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+# flat-doodle
+
+Cute, simple doodle illustrations with bold outlines
+
+## Color Palette
+
+- Primary: Bright pastel pink (#FFB6C1), mint (#98D8C8), lavender (#C8A2C8), butter yellow (#FFFACD)
+- Background: Clean white (#FFFFFF)
+- Accents: Bold black outlines, soft coral, sky blue
+
+## Visual Elements
+
+- Bold black outlines around all shapes
+- Simple flat shapes with no shading
+- Cute rounded proportions
+- Productivity-themed icons
+- Isolated elements on white background
+- Minimal decorative details
+
+## Typography
+
+- Simple, clean sans-serif or hand-lettering
+- Bold and easily readable
+
+## Best For
+
+Productivity content, SaaS articles, workflow tutorials, app features, beginner guides, casual business