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/visual-elements.md

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+# Visual Elements Library
+
+Icon and symbol vocabulary organized by topic. Use these as building blocks for cover compositions.
+
+## Tech & Development
+
+| Element | Use For |
+|---------|---------|
+| Code window / Terminal | Programming, development |
+| Gear / Cog | Engineering, settings, process |
+| Circuit board / Chip | Hardware, AI, computing |
+| Binary / Data stream | Data, algorithms |
+| API brackets `</>` | Web development, APIs |
+| Cloud | Cloud computing, SaaS |
+| Lock / Shield | Security, privacy |
+| Network nodes | Distributed systems, connections |
+
+## Ideas & Innovation
+
+| Element | Use For |
+|---------|---------|
+| Lightbulb | Ideas, insights, innovation |
+| Rocket | Launch, growth, startups |
+| Target / Bullseye | Goals, precision, focus |
+| Puzzle piece | Problem solving, integration |
+| Key | Solutions, access, unlocking |
+| Magnifying glass | Analysis, search, discovery |
+| Chart / Graph | Data, trends, growth |
+| Arrow / Path | Direction, journey, progress |
+
+## Communication & Collaboration
+
+| Element | Use For |
+|---------|---------|
+| Speech bubble | Communication, dialogue |
+| Chat dots `...` | Conversation, messaging |
+| Handshake | Partnership, agreement |
+| Team / Figures | Collaboration, community |
+| Mail / Envelope | Notifications, outreach |
+| Megaphone | Announcements, marketing |
+| Network / Web | Social, connections |
+
+## Nature & Growth
+
+| Element | Use For |
+|---------|---------|
+| Plant / Sprout | Growth, organic, sustainability |
+| Tree | Established, structure, branching |
+| Leaf | Eco, natural, fresh |
+| Sun / Rays | Energy, positivity, new beginnings |
+| Mountain | Challenge, achievement, scale |
+| Wave | Flow, change, rhythm |
+| Seed → Plant | Transformation, potential |
+
+## Tools & Actions
+
+| Element | Use For |
+|---------|---------|
+| Wrench / Hammer | Building, fixing, tools |
+| Pencil / Pen | Writing, creation, editing |
+| Brush | Design, creativity, art |
+| Scissors | Cutting, editing, trimming |
+| Clock / Timer | Time, scheduling, deadlines |
+| Calendar | Planning, events, milestones |
+| Checklist / Checkbox | Tasks, completion, validation |
+
+## Abstract Concepts
+
+| Element | Use For |
+|---------|---------|
+| Infinity ∞ | Continuous, endless, loops |
+| Yin-yang | Balance, duality, harmony |
+| Spiral | Evolution, recursion, cycles |
+| Stack / Layers | Depth, hierarchy, structure |
+| Bridge | Connection, transition, spanning |
+| Door / Portal | Opportunity, entry, access |
+| Mirror / Reflection | Self-improvement, analysis |
+
+## Combination Patterns
+
+Create visual metaphors by combining elements:
+
+| Combination | Represents |
+|-------------|------------|
+| Lightbulb + Gear | Innovative engineering |
+| Plant + Code | Organic tech growth |
+| Rocket + Target | Precise acceleration |
+| Key + Lock | Security solutions |
+| Bridge + People | Team connections |
+| Magnifier + Data | Analytics, insights |
+
+## Rendering-Specific Treatment
+
+| Rendering | Element Style |
+|-----------|---------------|
+| `flat-vector` | Geometric, simple shapes, uniform fills |
+| `hand-drawn` | Sketchy, organic, doodle-like |
+| `painterly` | Soft edges, brush strokes |
+| `digital` | Precise, gradient hints, polished |
+| `pixel` | 8-bit chunky, grid-aligned |
+| `chalk` | Dusty, textured, board style |