refactor(baoyu-cover-image): replace 20 styles with 5D palette×rendering system

Migrate from 4-dimension (Type×Style×Text×Mood) to 5-dimension
(Type×Palette×Rendering×Text×Mood) system. 9 palettes × 6 renderings
= 54 combinations replacing 20 fixed styles. Add style presets for
backward compatibility, v2→v3 schema migration, and new reference
structure (palettes/, renderings/, workflow/).

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

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+# Step 3: Prompt Template
+
+Save to `prompts/cover.md`:
+
+```markdown
+# Content Context
+Article title: [full original title from source]
+Content summary: [2-3 sentence summary of key points and themes]
+Keywords: [5-8 key terms extracted from content]
+
+# Visual Design
+Cover theme: [2-3 words visual interpretation]
+Type: [confirmed type]
+Palette: [confirmed palette]
+Rendering: [confirmed rendering]
+Text level: [confirmed text level]
+Mood: [confirmed mood]
+Aspect ratio: [confirmed ratio]
+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]"
+
+# Mood Application
+[Based on mood level:]
+- subtle: "Use low contrast, muted colors, light visual weight, calm aesthetic"
+- balanced: "Use medium contrast, normal saturation, balanced visual weight"
+- bold: "Use high contrast, vivid saturated colors, heavy visual weight, dynamic energy"
+
+# Composition
+Type composition:
+- [Type-specific layout and structure]
+
+Visual composition:
+- Main visual: [metaphor derived from content meaning]
+- Layout: [positioning based on type and aspect ratio]
+- Decorative: [palette-specific elements that reinforce content theme]
+
+Color scheme: [primary, background, accent from palette definition, adjusted by mood]
+Rendering notes: [key characteristics from rendering definition — lines, texture, depth, element style]
+Type notes: [key characteristics from type definition]
+Palette notes: [key characteristics from palette definition]
+
+[Watermark section if enabled]
+```
+
+## Content-Driven Design
+
+- Article title and summary inform the visual metaphor choice
+- Keywords guide decorative elements and symbols
+- The skill controls visual style; the content drives meaning
+
+## Type-Specific Composition
+
+| Type | Composition Guidelines |
+|------|------------------------|
+| `hero` | Large focal visual (60-70% area), title overlay on visual, dramatic composition |
+| `conceptual` | Abstract shapes representing core concepts, information hierarchy, clean zones |
+| `typography` | Title as primary element (40%+ area), minimal supporting visuals, strong hierarchy |
+| `metaphor` | Concrete object/scene representing abstract idea, symbolic elements, emotional resonance |
+| `scene` | Atmospheric environment, narrative elements, mood-setting lighting and colors |
+| `minimal` | Single focal element, generous whitespace (60%+), essential shapes only |
+
+## 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")
+- Match confirmed language
+
+## Watermark Application
+
+If enabled in preferences, add to prompt:
+
+```
+Include a subtle watermark "[content]" positioned at [position].
+The watermark should be legible but not distracting from the main content.
+```
+
+Reference: `config/watermark-guide.md`