feat(baoyu-cover-image): add font dimension with 4 styles

Add 6th dimension for typography control:
- clean: modern geometric sans-serif
- handwritten: warm hand-lettered style
- serif: classic elegant typography
- display: bold decorative headlines

Includes auto-selection rules, compatibility matrix, and warm-flat preset.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

skills/baoyu-cover-image/references/style-presets.md

@@ -22,6 +22,7 @@
 | `vector-illustration` | `retro` | `flat-vector` |
 | `vintage` | `retro` | `hand-drawn` |
 | `warm` | `warm` | `hand-drawn` |
+| `warm-flat` | `warm` | `flat-vector` |
 | `watercolor` | `earth` | `painterly` |
 
 ## Override Examples