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/dimensions/font.md

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+---
+name: font-dimension
+description: Typography style dimension for cover images
+---
+
+# Font Dimension
+
+Controls typography style and character feel.
+
+## Values
+
+| Font | Visual Style | Line Quality | Character |
+|------|--------------|--------------|-----------|
+| `clean` | Geometric sans-serif | Sharp, uniform | Modern, precise, neutral |
+| `handwritten` | Hand-lettered, brush | Organic, varied | Warm, personal, friendly |
+| `serif` | Classic serifs, elegant | Refined, structured | Editorial, authoritative |
+| `display` | Bold, decorative | Heavy, expressive | Attention-grabbing, playful |
+
+## Detail
+
+### clean
+
+Modern, universal typography with neutral character.
+
+**Characteristics**:
+- Geometric sans-serif letterforms
+- Sharp, uniform line weight
+- Clean edges, no flourishes
+- High readability at all sizes
+- Minimal personality, maximum clarity
+
+**Use Cases**:
+- Technical documentation
+- Professional/corporate content
+- Minimal design approaches
+- Data-driven articles
+- Modern brand aesthetics
+
+**Prompt Hints**:
+- Use clean geometric sans-serif typography
+- Modern, minimal letterforms
+- Sharp edges, uniform stroke weight
+- High contrast against background
+
+### handwritten
+
+Warm, organic typography with personal character.
+
+**Characteristics**:
+- Hand-lettered or brush style
+- Organic, varied line weight
+- Natural imperfections
+- Approachable, human feel
+- Casual yet intentional
+
+**Use Cases**:
+- Personal stories
+- Lifestyle content
+- Wellness and self-improvement
+- Creative tutorials
+- Friendly brand voices
+
+**Prompt Hints**:
+- Use warm hand-lettered typography with organic brush strokes
+- Friendly, personal feel
+- Natural variation in stroke weight
+- Approachable, human character
+
+### serif
+
+Classic, elegant typography with editorial authority.
+
+**Characteristics**:
+- Traditional serif letterforms
+- Refined, structured strokes
+- Elegant proportions
+- Timeless sophistication
+- Formal, trustworthy feel
+
+**Use Cases**:
+- Editorial content
+- Academic articles
+- Luxury brand content
+- Historical topics
+- Literary pieces
+
+**Prompt Hints**:
+- Use elegant serif typography with refined letterforms
+- Classic, editorial character
+- Structured, proportional spacing
+- Authoritative, sophisticated feel
+
+### display
+
+Bold, decorative typography for maximum impact.
+
+**Characteristics**:
+- Heavy, expressive letterforms
+- Decorative elements
+- Strong visual presence
+- Playful or dramatic character
+- Designed for headlines
+
+**Use Cases**:
+- Announcements
+- Entertainment content
+- Promotional materials
+- Event marketing
+- Gaming topics
+
+**Prompt Hints**:
+- Use bold decorative display typography
+- Heavy, expressive headlines
+- Strong visual impact
+- Attention-grabbing character
+
+## Default
+
+`clean` — Universal, pairs well with most rendering styles.
+
+## Rendering Compatibility
+
+| Font × Rendering | flat-vector | hand-drawn | painterly | digital | pixel | chalk |
+|------------------|:-----------:|:----------:|:---------:|:-------:|:-----:|:-----:|
+| clean | ✓✓ | ✗ | ✗ | ✓✓ | ✓ | ✗ |
+| handwritten | ✓ | ✓✓ | ✓✓ | ✓ | ✗ | ✓✓ |
+| serif | ✓ | ✗ | ✓ | ✓✓ | ✗ | ✗ |
+| display | ✓✓ | ✓ | ✓ | ✓✓ | ✓✓ | ✓ |
+
+✓✓ = highly recommended | ✓ = compatible | ✗ = not recommended
+
+## Type Compatibility
+
+| Font × Type | hero | conceptual | typography | metaphor | scene | minimal |
+|-------------|:----:|:----------:|:----------:|:--------:|:-----:|:-------:|
+| clean | ✓ | ✓✓ | ✓✓ | ✓ | ✗ | ✓✓ |
+| handwritten | ✓✓ | ✓ | ✓ | ✓✓ | ✓✓ | ✓ |
+| serif | ✓ | ✓ | ✓✓ | ✓ | ✓ | ✓ |
+| display | ✓✓ | ✓ | ✓✓ | ✓ | ✓ | ✗ |
+
+## Palette Interaction
+
+Font style adapts to palette characteristics:
+
+| Palette Category | clean | handwritten | serif | display |
+|------------------|-------|-------------|-------|---------|
+| Warm (warm, earth, pastel) | Softer weight | Natural fit | Warm tones | Playful energy |
+| Cool (cool, mono, elegant) | Perfect match | Contrast | Classic pairing | Bold statement |
+| Dark (dark, vivid) | High contrast | Glow effects | Dramatic | Maximum impact |
+| Vintage (retro) | Modern contrast | Nostalgic fit | Period-appropriate | Retro headlines |
+
+## Auto Selection
+
+When `--font` is omitted, select based on signals:
+
+| Signals | Font |
+|---------|------|
+| Personal, lifestyle, human, warm, friendly, story | `handwritten` |
+| Technical, professional, clean, modern, minimal, data | `clean` |
+| Editorial, academic, luxury, classic, literary | `serif` |
+| Announcement, entertainment, promotion, bold, event, gaming | `display` |
+
+Default: `clean`