feat(baoyu-cover-image): add reference image support and visual elements library

- Add --ref parameter for reference images (style/palette/direct usage)
- Add visual-elements.md with icon vocabulary by topic
- Enhance first-time setup as blocking operation
- Remove character limits from titles, use original source titles
- Update prompt template with reference handling

skills/baoyu-cover-image/references/workflow/prompt-template.md

@@ -3,6 +3,19 @@
 Save to `prompts/cover.md`:
 
 ```markdown
+---
+type: cover
+palette: [confirmed palette]
+rendering: [confirmed rendering]
+references:
+  - ref_id: 01
+    filename: refs/ref-01-{slug}.{ext}
+    usage: direct | style | palette
+  - ref_id: 02
+    filename: refs/ref-02-{slug}.{ext}
+    usage: direct | style | palette
+---
+
 # Content Context
 Article title: [full original title from source]
 Content summary: [2-3 sentence summary of key points and themes]
@@ -21,9 +34,9 @@ Language: [confirmed language]
 # Text Elements
 [Based on text level:]
 - none: "No text elements"
-- title-only: "Title: [max 8 chars headline]"
-- title-subtitle: "Title: [headline] / Subtitle: [max 15 chars context]"
-- text-rich: "Title: [headline] / Subtitle: [context] / Tags: [2-4 keywords]"
+- title-only: "Title: [exact title from source or user]"
+- title-subtitle: "Title: [title] / Subtitle: [context]"
+- text-rich: "Title: [title] / Subtitle: [context] / Tags: [2-4 keywords]"
 
 # Mood Application
 [Based on mood level:]
@@ -46,6 +59,8 @@ Type notes: [key characteristics from type definition]
 Palette notes: [key characteristics from palette definition]
 
 [Watermark section if enabled]
+
+[Reference images section if provided — see below]
 ```
 
 ## Content-Driven Design
@@ -54,6 +69,21 @@ Palette notes: [key characteristics from palette definition]
 - Keywords guide decorative elements and symbols
 - The skill controls visual style; the content drives meaning
 
+## Visual Element Selection
+
+Match content themes to icon vocabulary:
+
+| Content Theme | Suggested Elements |
+|---------------|-------------------|
+| Programming/Dev | Code window, terminal, API brackets, gear |
+| AI/ML | Brain, neural network, robot, circuit |
+| Growth/Business | Chart, rocket, plant, mountain, arrow |
+| Security | Lock, shield, key, fingerprint |
+| Communication | Speech bubble, megaphone, mail, handshake |
+| Tools/Methods | Wrench, checklist, pencil, puzzle |
+
+Full library: [../visual-elements.md](../visual-elements.md)
+
 ## Type-Specific Composition
 
 | Type | Composition Guidelines |
@@ -68,8 +98,8 @@ Palette notes: [key characteristics from palette definition]
 ## Title Guidelines
 
 When text level includes title:
-- Max 8 characters, punchy headline
-- Use engagement hooks: numbers ("3个陷阱"), questions ("Why X?"), contrasts ("A vs B"), pain points ("别再X了")
+- **Source**: Use the exact title provided by user, or extract from source content
+- **Do NOT invent titles**: Stay faithful to the original
 - Match confirmed language
 
 ## Watermark Application
@@ -82,3 +112,118 @@ The watermark should be legible but not distracting from the main content.
 ```
 
 Reference: `config/watermark-guide.md`
+
+## Reference Image Handling
+
+When user provides reference images (`--ref` or pasted images):
+
+### ⚠️ CRITICAL - Frontmatter References
+
+**MUST add `references` field in YAML frontmatter** when reference files are saved to `refs/`:
+
+```yaml
+---
+type: cover
+palette: warm
+rendering: flat-vector
+references:
+  - ref_id: 01
+    filename: refs/ref-01-podcast-thumbnail.jpg
+    usage: style
+---
+```
+
+| Field | Description |
+|-------|-------------|
+| `ref_id` | Sequential number (01, 02, ...) |
+| `filename` | Relative path from prompt file's parent directory |
+| `usage` | `direct` / `style` / `palette` |
+
+**Omit `references` field entirely** if no reference files saved (style extracted verbally only).
+
+### When to Include References in Frontmatter
+
+| Situation | Frontmatter Action | Generation Action |
+|-----------|-------------------|-------------------|
+| Reference file saved to `refs/` | Add to `references` list ✓ | Pass via `--ref` parameter |
+| Style extracted verbally (no file) | Omit `references` field | Describe in prompt body only |
+| File path in frontmatter but doesn't exist | ERROR - fix or remove | Generation will fail |
+
+**Before writing prompt with references, verify**: `test -f refs/ref-NN-{slug}.{ext}`
+
+### Reference Usage Types
+
+| Usage | When to Use | Generation Action |
+|-------|-------------|-------------------|
+| `direct` | Reference matches desired output closely | Pass to `--ref` parameter |
+| `style` | Extract visual style characteristics only | Describe style in prompt text |
+| `palette` | Extract color palette only | Include colors in prompt |
+
+### Step 1: Analyze References
+
+For each reference image, extract:
+- **Style**: Rendering technique, line quality, texture
+- **Composition**: Layout, visual hierarchy, focal points
+- **Color mood**: Palette characteristics (without specific colors)
+- **Elements**: Key visual elements and symbols used
+
+### Step 2: Embed in Prompt
+
+**If file saved AND skill supports `--ref`** (e.g., baoyu-image-gen with Google):
+- Pass ref images directly via `--ref` parameter
+- Add brief style note in prompt:
+
+```
+# Reference Style
+Follow the visual style of the attached reference image(s):
+- [1-2 sentence style summary from analysis]
+```
+
+**If file saved BUT skill does NOT support `--ref`**:
+- Embed detailed text description in prompt:
+
+```
+# Reference Style (Text Description)
+Emulate the following visual style from reference image(s):
+
+Reference 1: [filename]
+- Style: [detailed rendering technique description]
+- Composition: [layout and hierarchy description]
+- Elements: [key visual elements used]
+- Mood: [emotional tone and visual weight]
+
+[Repeat for additional references]
+
+Apply these style characteristics to the cover design while maintaining the specified Type, Palette, and Rendering dimensions.
+```
+
+**If style/palette extracted verbally (NO file saved)**:
+- DO NOT add references metadata to prompt
+- Append extracted info directly to prompt body:
+
+```
+# Extracted Style (from reference)
+
+COLORS (from reference):
+- Primary: #E8756D coral
+- Secondary: #7ECFC0 mint
+...
+
+STYLE (from reference):
+- Clean lines, minimal shadows
+- Gradient backgrounds
+- Flat vector icons
+...
+```
+
+### Reference Analysis Template
+
+Use this format when analyzing reference images:
+
+| Aspect | Analysis Points |
+|--------|-----------------|
+| **Rendering** | Line quality (clean/sketchy/painted), texture presence, depth treatment |
+| **Composition** | Element placement, whitespace ratio, visual flow |
+| **Typography** | Font style, text placement, hierarchy (if present) |
+| **Elements** | Icon vocabulary, decorative motifs, character style |
+| **Mood** | Contrast level, saturation, visual weight |