feat(baoyu-xhs-images): add reference image chain for visual consistency

When generating multi-image series, use image 1 as --ref for all
subsequent images. This anchors character design, color rendering,
and illustration style across the entire series — critical for styles
with recurring characters or mascots.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills/baoyu-xhs-images/references/workflows/prompt-assembly.md

@@ -160,7 +160,21 @@ From outline entry, format:
 If preferences include watermark:
 - Add watermark section with content, position, opacity
 
-### Step 5: Combine
+### Step 5: Visual Consistency — Reference Image Chain
+
+When generating multiple images in a series:
+
+1. **Image 1 (cover)**: Generate without `--ref` — this establishes the visual anchor
+2. **Images 2+**: Always pass image 1 as `--ref` to the image generation skill:
+   ```bash
+   npx -y bun ${SKILL_DIR}/scripts/main.ts \
+     --promptfiles prompts/02-content-xxx.md \
+     --ref path/to/01-cover-xxx.png \
+     --image 02-content-xxx.png --ar 3:4 --quality 2k
+   ```
+   This ensures the AI maintains the same character design, illustration style, and color rendering across the series.
+
+### Step 6: Combine
 
 Assemble all sections into final prompt following base structure.