feat(baoyu-cover-image): add reference image support and visual elements library

- Add --ref parameter for reference images (style/palette/direct usage)
- Add visual-elements.md with icon vocabulary by topic
- Enhance first-time setup as blocking operation
- Remove character limits from titles, use original source titles
- Update prompt template with reference handling

skills/baoyu-cover-image/references/config/preferences-schema.md

@@ -103,8 +103,8 @@ custom_palettes:
 | Value | Description |
 |-------|-------------|
 | `none` | Pure visual, no text elements |
-| `title-only` | Single headline (≤8 characters) |
-| `title-subtitle` | Title + subtitle (≤15 characters) |
+| `title-only` | Single headline |
+| `title-subtitle` | Title + subtitle |
 | `text-rich` | Title + subtitle + keyword tags (2-4) |
 
 ## Mood Options