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/digital.md

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+# digital
+
+Polished, precise, modern
+
+## Core Characteristics
+
+Clean digital illustration with polished finish, precise edges, and subtle modern effects. Feels like a professional UI mockup or corporate illustration.
+
+## Lines
+
+- Clean, precise, computer-perfect edges
+- Consistent stroke weights
+- Sharp corners where appropriate
+- Anti-aliased smooth rendering
+
+## Texture
+
+- Smooth surfaces with no visible texture
+- Subtle gradients permitted (soft, controlled)
+- Frosted glass and blur effects
+- Clean shadows with consistent direction
+
+## Depth
+
+- Subtle gradients and soft drop shadows
+- Layered card-based layouts
+- Light 3D effects (subtle, not realistic)
+- Material Design-inspired elevation
+
+## Element Vocabulary
+
+- Polished icons and UI components
+- Data visualizations: charts, graphs, metrics
+- Card layouts and structured grids
+- Tag chips, progress bars, status indicators
+- Clean geometric shapes
+
+## Typography Approach
+
+- System UI or modern sans-serif (Inter, SF Pro style)
+- Clean, functional, high readability
+- Structured hierarchy with consistent spacing