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/earth.md

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+# earth
+
+Natural, organic, grounded
+
+## Color Palette
+
+| Role | Color | Hex |
+|------|-------|-----|
+| Primary 1 | Forest Green | #276749 |
+| Primary 2 | Sage | #9AE6B4 |
+| Primary 3 | Earth Brown | #744210 |
+| Background | Sand Beige | #F5E6D3 |
+| Background Alt | Sky Blue | #E0F2FE |
+| Accent 1 | Sunset Orange | #ED8936 |
+| Accent 2 | Water Blue | #63B3ED |
+
+## Decorative Hints
+
+- Leaves, trees, mountains, natural forms
+- Sun, clouds, organic flowing lines
+- Botanical illustrations
+- Earthy textures and natural patterns
+
+## Best For
+
+Nature, wellness, eco, organic, travel, sustainability, outdoor topics, slow living