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/retro.md

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+# retro
+
+Nostalgic, vintage, classic
+
+## Color Palette
+
+| Role | Color | Hex |
+|------|-------|-----|
+| Primary 1 | Coral Red | #E07A5F |
+| Primary 2 | Mint Green | #81B29A |
+| Primary 3 | Mustard Yellow | #F2CC8F |
+| Primary 4 | Dark Maroon | #5D3A3A |
+| Background | Cream Off-White | #F5F0E6 |
+| Background Alt | Aged Paper | #F5E6D3 |
+| Accent 1 | Burnt Orange | #D4764A |
+| Accent 2 | Rock Blue | #577590 |
+| Accent 3 | Vintage Gold | #C9A227 |
+| Accent 4 | Faded Teal | #2F7373 |
+
+## Decorative Hints
+
+- Halftone dots and vintage badges
+- Aged textures with subtle paper grain
+- Sunburst/radiating lines for energy
+- Pill-shaped clouds, small dots and stars
+- Classic icons and retro motifs
+
+## Best For
+
+History, vintage, retro, classic, exploration, retrospectives, throwback content, creative proposals, educational