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/mono.md

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+# mono
+
+Clean, focused, essential
+
+## Color Palette
+
+| Role | Color | Hex |
+|------|-------|-----|
+| Primary 1 | Pure Black | #000000 |
+| Primary 2 | Near Black | #1F1F1F |
+| Primary 3 | Dark Gray | #374151 |
+| Background | White | #FFFFFF |
+| Background Alt | Off-White | #FAFAFA |
+| Accent 1 | Content-derived single color | - |
+| Accent 2 | Medium Gray | #9CA3AF |
+
+## Decorative Hints
+
+- Maximum negative space
+- Thin lines and minimal strokes
+- Single focal point emphasis
+- Stark contrast between elements
+
+## Best For
+
+Zen, focus, essential concepts, pure, simple, minimalist philosophy, clean design