refactor(baoyu-article-illustrator): extract content to reference files

- Simplify SKILL.md by extracting usage and prompt templates
- Add references/usage.md for command syntax
- Add references/prompt-construction.md for prompt templates
- Add default_output_dir preference option
- Reorganize workflow from 5 to 6 steps with Pre-check phase

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

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+# Prompt Construction
+
+## Principles
+
+Good prompts must include:
+
+1. **Layout Structure First**: Describe composition, zones, flow direction
+2. **Specific Data/Labels**: Use actual numbers, terms from article
+3. **Visual Relationships**: How elements connect
+4. **Semantic Colors**: Meaning-based color choices (red=warning, green=efficient)
+5. **Style Characteristics**: Line treatment, texture, mood
+6. **Aspect Ratio**: End with ratio and complexity level
+
+## Type-Specific Templates
+
+### Infographic
+
+```
+[Title] - Data Visualization
+
+Layout: [grid/radial/hierarchical]
+
+ZONES:
+- Zone 1: [data point with specific values]
+- Zone 2: [comparison with metrics]
+- Zone 3: [summary/conclusion]
+
+LABELS: [specific numbers, percentages, terms from article]
+COLORS: [semantic color mapping]
+STYLE: [style characteristics]
+ASPECT: 16:9
+```
+
+### Scene
+
+```
+[Title] - Atmospheric Scene
+
+FOCAL POINT: [main subject]
+ATMOSPHERE: [lighting, mood, environment]
+MOOD: [emotion to convey]
+COLOR TEMPERATURE: [warm/cool/neutral]
+STYLE: [style characteristics]
+ASPECT: 16:9
+```
+
+### Flowchart
+
+```
+[Title] - Process Flow
+
+Layout: [left-right/top-down/circular]
+
+STEPS:
+1. [Step name] - [brief description]
+2. [Step name] - [brief description]
+...
+
+CONNECTIONS: [arrow types, decision points]
+STYLE: [style characteristics]
+ASPECT: 16:9
+```
+
+### Comparison
+
+```
+[Title] - Comparison View
+
+LEFT SIDE - [Option A]:
+- [Point 1]
+- [Point 2]
+
+RIGHT SIDE - [Option B]:
+- [Point 1]
+- [Point 2]
+
+DIVIDER: [visual separator]
+STYLE: [style characteristics]
+ASPECT: 16:9
+```
+
+### Framework
+
+```
+[Title] - Conceptual Framework
+
+STRUCTURE: [hierarchical/network/matrix]
+
+NODES:
+- [Concept 1] - [role]
+- [Concept 2] - [role]
+
+RELATIONSHIPS: [how nodes connect]
+STYLE: [style characteristics]
+ASPECT: 16:9
+```
+
+### Timeline
+
+```
+[Title] - Chronological View
+
+DIRECTION: [horizontal/vertical]
+
+EVENTS:
+- [Date/Period 1]: [milestone]
+- [Date/Period 2]: [milestone]
+
+MARKERS: [visual indicators]
+STYLE: [style characteristics]
+ASPECT: 16:9
+```
+
+## What to Avoid
+
+- Vague descriptions ("a nice image")
+- Literal metaphor illustrations
+- Missing concrete labels/annotations
+- Generic decorative elements
+
+## Watermark Integration
+
+If watermark enabled in preferences, append:
+
+```
+Include a subtle watermark "[content]" positioned at [position] with approximately [opacity*100]% visibility.
+```