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/style-presets.md

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+# Style Presets
+
+`--style X` expands to a palette + rendering combination. Users can override either dimension.
+
+| --style | Palette | Rendering |
+|---------|---------|-----------|
+| `elegant` | `elegant` | `hand-drawn` |
+| `blueprint` | `cool` | `digital` |
+| `chalkboard` | `dark` | `chalk` |
+| `dark-atmospheric` | `dark` | `digital` |
+| `editorial-infographic` | `cool` | `digital` |
+| `fantasy-animation` | `pastel` | `painterly` |
+| `flat-doodle` | `pastel` | `flat-vector` |
+| `intuition-machine` | `retro` | `digital` |
+| `minimal` | `mono` | `flat-vector` |
+| `nature` | `earth` | `hand-drawn` |
+| `notion` | `mono` | `digital` |
+| `pixel-art` | `vivid` | `pixel` |
+| `playful` | `pastel` | `hand-drawn` |
+| `retro` | `retro` | `digital` |
+| `sketch-notes` | `warm` | `hand-drawn` |
+| `vector-illustration` | `retro` | `flat-vector` |
+| `vintage` | `retro` | `hand-drawn` |
+| `warm` | `warm` | `hand-drawn` |
+| `watercolor` | `earth` | `painterly` |
+
+## Override Examples
+
+- `--style blueprint --rendering hand-drawn` = cool palette with hand-drawn rendering
+- `--style elegant --palette warm` = warm palette with hand-drawn rendering
+
+Explicit `--palette`/`--rendering` flags always override preset values.