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/renderings/hand-drawn.md

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+# hand-drawn
+
+Sketchy, organic, personal
+
+## Core Characteristics
+
+Hand-drawn illustration with visible imperfections, organic line quality, and personal touch. Feels like a skilled artist's sketchbook or whiteboard drawing.
+
+## Lines
+
+- Sketchy, organic, slightly imperfect strokes
+- Variable line weight (thicker at pressure points)
+- Wavy connectors and arrows
+- Natural hand tremor visible
+
+## Texture
+
+- Paper grain and subtle surface texture
+- Pencil/pen/marker texture on strokes
+- Casual fills with visible brush direction
+
+## Depth
+
+- Minimal: light hand-drawn shadows or hatching
+- No realistic depth or perspective
+- Simple layering with overlap
+
+## Element Vocabulary
+
+- Doodles, organic shapes, hand-lettered labels
+- Conceptual icons with sketchy quality
+- Connection lines with hand-drawn wavy feel
+- Stars, arrows, underlines, circles, checkmarks
+- Stick figures and simple characters
+
+## Typography Approach
+
+- Hand-lettered or marker-style text
+- Bouncy baselines, organic feel
+- Variable sizes for emphasis hierarchy