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/elegant.md

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+# elegant
+
+Sophisticated, refined, understated luxury
+
+## Color Palette
+
+| Role | Color | Hex |
+|------|-------|-----|
+| Primary 1 | Soft Coral | #E8A598 |
+| Primary 2 | Muted Teal | #5B8A8A |
+| Primary 3 | Dusty Rose | #D4A5A5 |
+| Background | Warm Cream | #F5F0E6 |
+| Background Alt | Soft Beige | #F0EBE0 |
+| Accent 1 | Gold | #C9A962 |
+| Accent 2 | Copper | #B87333 |
+
+## Decorative Hints
+
+- Delicate ornamental details
+- Subtle gradients and soft transitions
+- Refined geometric patterns
+- Balanced, symmetrical compositions
+
+## Best For
+
+Business, professional, thought leadership, luxury, corporate communications