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/warm.md

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+# warm
+
+Friendly, approachable, human-centered
+
+## Color Palette
+
+| Role | Color | Hex |
+|------|-------|-----|
+| Primary 1 | Warm Orange | #ED8936 |
+| Primary 2 | Golden Yellow | #F6AD55 |
+| Primary 3 | Terracotta | #C05621 |
+| Background | Cream | #FFFAF0 |
+| Background Alt | Soft Peach | #FED7AA |
+| Accent 1 | Deep Brown | #744210 |
+| Accent 2 | Soft Red | #E53E3E |
+
+## Decorative Hints
+
+- Sun rays, warm lighting effects
+- Rounded shapes, organic curves
+- Hearts, smiling faces, friendly icons
+- Warm gradient overlays
+
+## Best For
+
+Personal growth, lifestyle, education, human stories, emotion, community