feat: add performance optimization skill

[ryzizub]

Description

Add a skill that identifies and fixes common Flutter performance issues. The skill should guide through diagnosing performance bottlenecks using Flutter DevTools and applying targeted optimizations.

Areas to cover:

• Widget rebuild minimization — const constructors, splitting widgets, RepaintBoundary, selective rebuilds with ValueListenableBuilder/BlocSelector
• List & scroll performance — ListView.builder/GridView.builder for lazy loading, itemExtent for fixed-height lists, AutomaticKeepAliveClientMixin trade-offs
• Image optimization — caching with CachedNetworkImage, proper cacheWidth/cacheHeight for decode-time savings, precaching
• Build & render pipeline — identifying expensive layouts, avoiding Opacity widget for hiding (use Visibility), minimizing saveLayer triggers
• Debug tooling — using Flutter DevTools (Performance overlay, Widget rebuild tracker, Timeline), debugPrintRebuildDirtyWidgets, debugProfileBuildsEnabled, profile mode vs debug mode for accurate measurements
• Shader compilation jank — warm-up strategies with ShaderWarmUp, --cache-sksl for SkSL shaders
• Async & isolate patterns — offloading heavy computation to isolates via Isolate.run/compute, debouncing rapid state changes
• Common anti-patterns — building widgets inside build() methods, unnecessary setState scope, using GlobalKey excessively, deep widget trees

Requirements

• All CI/CD checks are passing.
• There is no drop in the test coverage percentage.

Additional Context

This skill complements the existing vgv-bloc (state management efficiency), vgv-testing (golden tests for visual regressions), and vgv-ui-package (reusable widget design) skills by focusing on runtime performance characteristics.