refactor(baoyu-cover-image): replace 20 styles with 5D palette×rendering system

Migrate from 4-dimension (Type×Style×Text×Mood) to 5-dimension
(Type×Palette×Rendering×Text×Mood) system. 9 palettes × 6 renderings
= 54 combinations replacing 20 fixed styles. Add style presets for
backward compatibility, v2→v3 schema migration, and new reference
structure (palettes/, renderings/, workflow/).

skills/baoyu-cover-image/references/palettes/pastel.md

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+# pastel
+
+Gentle, whimsical, soft
+
+## Color Palette
+
+| Role | Color | Hex |
+|------|-------|-----|
+| Primary 1 | Soft Pink | #FFB6C1 |
+| Primary 2 | Mint | #98D8C8 |
+| Primary 3 | Lavender | #C8A2C8 |
+| Background | White | #FFFFFF |
+| Background Alt | Light Cream | #FFF8E7 |
+| Accent 1 | Butter Yellow | #FFFACD |
+| Accent 2 | Sky Blue | #BEE3F8 |
+
+## Decorative Hints
+
+- Cute rounded proportions
+- Stars, sparkles, flowers, decorative flourishes
+- Soft shadows and gentle highlights
+- Storybook-style elements
+
+## Best For
+
+Fantasy, children, gentle content, creative, whimsical, casual, beginner guides