Add comprehensive Skills documentation for G2 visualization library

[Copilot]

Created structured knowledge base to help AI assistants understand and work with the G2 codebase effectively.

Changes

Added 7 Skills files in .github/skills/ covering G2's architecture:

• g2-overview.md - Library system (litelib/corelib/plotlib/etc), API structure, file organization, core patterns
• working-with-marks.md - 35+ mark types (interval/line/point/treemap/sankey/etc), channels, shapes, operations
• transforms-and-data.md - Data transforms (fetch/filter/map/kde) and mark transforms (stackY/dodgeX/binX/groupX)
• scales-coordinates-encoding.md - Scale types (linear/log/ordinal/band), coordinate systems (polar/theta/radar/helix), visual encoding
• interactions-animations.md - Interactions (highlight/select/filter/brush/tooltip) and animations (fadeIn/scaleIn/morphing)
• api-usage-extension.md - Fluent vs Spec API, Chart/Mark/Composition classes, register() extension system, custom components
• components-compositions-themes.md - UI components (axes/legends/sliders), compositions (layer/facet/matrix), theme system
• README.md - Skills overview and usage guide

Structure

Each skill includes:

• Source file references (src/mark/interval.ts)
• TypeScript code examples
• Common patterns and usage
• Performance considerations

Example

// Skills document how to extend G2
import { register } from '@antv/g2';

register('mark.custom', customMark);
register('transform.custom', customTransform);

chart.custom().transform({ type: 'custom' });

Skills enable AI assistants to navigate G2's grammar-of-graphics architecture, understand the library/mark/transform/scale system, and follow extension patterns.

Original prompt

仔细阅读 G2 源码，提供 Skills ，以便扩展 Claude 的能力。

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