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/cool.md

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+# cool
+
+Technical, professional, precise
+
+## Color Palette
+
+| Role | Color | Hex |
+|------|-------|-----|
+| Primary 1 | Engineering Blue | #2563EB |
+| Primary 2 | Navy Blue | #1E3A5F |
+| Primary 3 | Cyan | #06B6D4 |
+| Background | Light Gray | #F8F9FA |
+| Background Alt | Blueprint Off-White | #FAF8F5 |
+| Accent 1 | Amber | #F59E0B |
+| Accent 2 | Light Blue | #BFDBFE |
+
+## Decorative Hints
+
+- Grid lines and alignment guides
+- Dimension indicators and measurements
+- Technical schematics and diagrams
+- Geometric precision elements
+
+## Best For
+
+Architecture, system design, API, technical documentation, engineering, data analysis