feat(baoyu-comic): add four-panel preset, minimalist art style, and flexible image workflow

- Add minimalist art style (clean line art, limited spot color, stick-figure characters)
- Add four-panel layout with 起承转合 structure
- Add four-panel preset combining minimalist + neutral + four-panel layout
- Make character sheet conditional (skip for simple presets like four-panel)
- Add --ref failure recovery: compress → retry → fall back to prompt-only

skills/baoyu-comic/references/layouts/four-panel.md

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+# four-panel
+
+四格漫画 - Strict 2×2 grid, single-page story
+
+## Panel Structure
+
+- **Panels per page**: 4 (exactly, no variation)
+- **Structure**: Strict 2×2 equal grid
+- **Gutters**: Consistent white space (8-10px), uniform on all sides
+
+## Grid Configuration
+
+- 2 columns × 2 rows, all panels identical size
+- Panel sizes: Exactly equal (each panel = 25% of content area)
+- Reading flow: Z-pattern — Panel 1 (top-left) → Panel 2 (top-right) → Panel 3 (bottom-left) → Panel 4 (bottom-right)
+
+## Narrative Structure
+
+Each panel serves a specific narrative role (起承转合 / kishōtenketsu):
+
+| Panel | Position | Role | Purpose |
+|-------|----------|------|---------|
+| 1 | Top-left | 起 Setup | Establish situation, introduce characters/problem |
+| 2 | Top-right | 承 Development | Build on setup, add complication or attempt |
+| 3 | Bottom-left | 转 Turn | Twist, key insight, or reversal — the pivotal moment |
+| 4 | Bottom-right | 合 Conclusion | Resolution, punchline, or takeaway |
+
+## Aspect Ratio
+
+- Recommended page aspect: **4:3** (landscape)
+- Landscape gives each panel a comfortable wide rectangle
+- Portrait (3:4) makes panels tall and narrow — avoid for this layout
+
+## Best For
+
+Business allegory, quick-insight education, social media comics, fables, parables, single-concept explanation
+
+## Best Style Pairings
+
+minimalist, ligne-claire, chalk