feat(skills): expand app-builder skill with comprehensive p5.js, Three.js, and dashboard guidance
Rewrites the app-builder SKILL.md from ~250 lines to ~2300 lines covering
the full Dench App system: p5.js for 2D games (state machines, sprites,
particles, physics, tilemaps, sound), Three.js for 3D (scene setup, controls,
GLTF loading, post-processing, first-person patterns), data dashboards
(Chart.js, D3, CSS-only), interactive tools, multi-file organization,
asset management, theme integration, error handling, and performance tuning.
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