refactor(baoyu-article-illustrator): simplify SKILL.md and add Core Styles tier

- Extract detailed workflow to references/workflow.md
- Add Core Styles for quick selection (vector, minimal-flat, sci-fi, hand-drawn, editorial, scene)
- Add vector-illustration as recommended default style
- Add Illustration Purpose (information/visualization/imagination)
- Add default composition, character rendering, text styling to prompts

skills/baoyu-article-illustrator/references/prompt-construction.md

@@ -49,6 +49,54 @@ STYLE (from reference):
 
 ---
 
+## Default Composition Requirements
+
+**Apply to ALL prompts by default**:
+
+| Requirement | Description |
+|-------------|-------------|
+| **Clean composition** | Simple layouts, no visual clutter |
+| **White space** | Generous margins, breathing room around elements |
+| **No complex backgrounds** | Solid colors or subtle gradients only, avoid busy textures |
+| **Centered or content-appropriate** | Main visual elements centered or positioned by content needs |
+| **Matching graphics** | Use graphic elements that align with content theme |
+| **Highlight core info** | White space draws attention to key information |
+
+**Add to ALL prompts**:
+> Clean composition with generous white space. Simple or no background. Main elements centered or positioned by content needs.
+
+---
+
+## Character Rendering
+
+When depicting people:
+
+| Guideline | Description |
+|-----------|-------------|
+| **Style** | Simplified cartoon silhouettes or symbolic expressions |
+| **Avoid** | Realistic human portrayals, detailed faces |
+| **Diversity** | Varied body types when showing multiple people |
+| **Emotion** | Express through posture and simple gestures |
+
+**Add to ALL prompts with human figures**:
+> Human figures: simplified stylized silhouettes or symbolic representations, not photorealistic.
+
+---
+
+## Text in Illustrations
+
+| Element | Guideline |
+|---------|-----------|
+| **Size** | Large, prominent, immediately readable |
+| **Style** | Handwritten fonts preferred for warmth |
+| **Content** | Concise keywords and core concepts only |
+| **Language** | Match article language |
+
+**Add to prompts with text**:
+> Text should be large and prominent with handwritten-style fonts. Keep minimal, focus on keywords.
+
+---
+
 ## Principles
 
 Good prompts must include:
@@ -80,6 +128,13 @@ STYLE: [style characteristics]
 ASPECT: 16:9
 ```
 
+**Infographic + vector-illustration**:
+```
+Flat vector illustration infographic. Clean black outlines on all elements.
+COLORS: Cream background (#F5F0E6), Coral Red (#E07A5F), Mint Green (#81B29A), Mustard Yellow (#F2CC8F)
+ELEMENTS: Geometric simplified icons, no gradients, playful decorative elements (dots, stars)
+```
+
 ### Scene
 
 ```
@@ -110,6 +165,13 @@ STYLE: [style characteristics]
 ASPECT: 16:9
 ```
 
+**Flowchart + vector-illustration**:
+```
+Flat vector flowchart with bold arrows and geometric step containers.
+COLORS: Cream background (#F5F0E6), steps in Coral/Mint/Mustard, black outlines
+ELEMENTS: Rounded rectangles, thick arrows, simple icons per step
+```
+
 ### Comparison
 
 ```
@@ -128,6 +190,13 @@ STYLE: [style characteristics]
 ASPECT: 16:9
 ```
 
+**Comparison + vector-illustration**:
+```
+Flat vector comparison with split layout. Clear visual separation.
+COLORS: Left side Coral (#E07A5F), Right side Mint (#81B29A), cream background
+ELEMENTS: Bold icons, black outlines, centered divider line
+```
+
 ### Framework
 
 ```
@@ -144,6 +213,13 @@ STYLE: [style characteristics]
 ASPECT: 16:9
 ```
 
+**Framework + vector-illustration**:
+```
+Flat vector framework diagram with geometric nodes and bold connectors.
+COLORS: Cream background (#F5F0E6), nodes in Coral/Mint/Mustard/Blue, black outlines
+ELEMENTS: Rounded rectangles or circles for nodes, thick connecting lines
+```
+
 ### Timeline
 
 ```