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/usage.md

@@ -0,0 +1,66 @@
+# Usage
+
+## Command Syntax
+
+```bash
+# Auto-select type and style based on content
+/baoyu-article-illustrator path/to/article.md
+
+# Specify type
+/baoyu-article-illustrator path/to/article.md --type infographic
+
+# Specify style
+/baoyu-article-illustrator path/to/article.md --style blueprint
+
+# Combine type and style
+/baoyu-article-illustrator path/to/article.md --type flowchart --style notion
+
+# Specify density
+/baoyu-article-illustrator path/to/article.md --density rich
+
+# Direct content input (paste mode)
+/baoyu-article-illustrator
+[paste content]
+```
+
+## Options
+
+| Option | Description |
+|--------|-------------|
+| `--type <name>` | Illustration type (see Type Gallery in SKILL.md) |
+| `--style <name>` | Visual style (see references/styles.md) |
+| `--density <level>` | Image count: minimal / balanced / rich |
+
+## Input Modes
+
+| Mode | Trigger | Output Directory |
+|------|---------|------------------|
+| File path | `path/to/article.md` | Use `default_output_dir` preference, or ask if not set |
+| Paste content | No path argument | `illustrations/{topic-slug}/` |
+
+## Output Directory Options
+
+| Value | Path |
+|-------|------|
+| `same-dir` | `{article-dir}/` |
+| `illustrations-subdir` | `{article-dir}/illustrations/` |
+| `independent` | `illustrations/{topic-slug}/` |
+
+Configure in EXTEND.md: `default_output_dir: illustrations-subdir`
+
+## Examples
+
+**Technical article with data**:
+```bash
+/baoyu-article-illustrator api-design.md --type infographic --style blueprint
+```
+
+**Personal story**:
+```bash
+/baoyu-article-illustrator journey.md --type scene --style warm
+```
+
+**Tutorial with steps**:
+```bash
+/baoyu-article-illustrator how-to-deploy.md --type flowchart --density rich
+```