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/compatibility.md

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+# Compatibility Matrices
+
+✓✓ = highly recommended | ✓ = compatible | ✗ = not recommended
+
+## Palette × Rendering
+
+| | flat-vector | hand-drawn | painterly | digital | pixel | chalk |
+|---|:---:|:---:|:---:|:---:|:---:|:---:|
+| warm | ✓✓ | ✓✓ | ✓ | ✓ | ✓ | ✓ |
+| elegant | ✓ | ✓✓ | ✓ | ✓✓ | ✗ | ✗ |
+| cool | ✓✓ | ✓ | ✗ | ✓✓ | ✓ | ✓ |
+| dark | ✓ | ✓ | ✓ | ✓✓ | ✓ | ✓✓ |
+| earth | ✓ | ✓✓ | ✓✓ | ✓ | ✗ | ✗ |
+| vivid | ✓✓ | ✓ | ✓ | ✓ | ✓✓ | ✓ |
+| pastel | ✓✓ | ✓✓ | ✓✓ | ✓ | ✗ | ✗ |
+| mono | ✓✓ | ✓ | ✗ | ✓✓ | ✓ | ✓ |
+| retro | ✓✓ | ✓✓ | ✓ | ✓✓ | ✓ | ✗ |
+
+## Type × Rendering
+
+| | flat-vector | hand-drawn | painterly | digital | pixel | chalk |
+|---|:---:|:---:|:---:|:---:|:---:|:---:|
+| hero | ✓ | ✓✓ | ✓✓ | ✓✓ | ✓ | ✓ |
+| conceptual | ✓✓ | ✓ | ✗ | ✓✓ | ✓ | ✓ |
+| typography | ✓✓ | ✓ | ✓ | ✓✓ | ✓ | ✓ |
+| metaphor | ✓ | ✓✓ | ✓✓ | ✓ | ✗ | ✓ |
+| scene | ✗ | ✓ | ✓✓ | ✓ | ✓ | ✗ |
+| minimal | ✓✓ | ✓ | ✓ | ✓✓ | ✗ | ✗ |
+
+## Type × Text
+
+| | none | title-only | title-subtitle | text-rich |
+|---|:---:|:---:|:---:|:---:|
+| hero | ✓ | ✓✓ | ✓✓ | ✓ |
+| conceptual | ✓✓ | ✓✓ | ✓ | ✓ |
+| typography | ✗ | ✓ | ✓✓ | ✓✓ |
+| metaphor | ✓✓ | ✓ | ✓ | ✗ |
+| scene | ✓✓ | ✓ | ✓ | ✗ |
+| minimal | ✓✓ | ✓✓ | ✓ | ✗ |
+
+## Type × Mood
+
+| | subtle | balanced | bold |
+|---|:---:|:---:|:---:|
+| hero | ✓ | ✓✓ | ✓✓ |
+| conceptual | ✓✓ | ✓✓ | ✓ |
+| typography | ✓ | ✓✓ | ✓✓ |
+| metaphor | ✓✓ | ✓✓ | ✓ |
+| scene | ✓✓ | ✓✓ | ✓ |
+| minimal | ✓✓ | ✓✓ | ✗ |