feat: animations skill

[B0berman]

Description

• Add a new vgv-animations skill covering Flutter animation best practices using the built-in animation framework and Material 3 motion guidelines
• Adds four reference files: animated-switcher.md, looping-animations.md, page-transitions.md, and staggered-animations.md

The skill guides developers through:

• Choosing the right animation type — decision tree from implicit (AnimatedFoo) to explicit (AnimationController) to hero/route transitions
• Material 3 motion tokens — enforces use of Durations and Easing constants instead of hardcoded values
• Implicit animations — AnimatedContainer, AnimatedOpacity, TweenAnimationBuilder, etc.
• Explicit animations — AnimationController lifecycle, TickerProviderStateMixin, chained tweens
• Page transitions — custom PageRouteBuilder and go_router integration
• Staggered animations — Interval-based sequencing patterns
• Anti-patterns — common mistakes like unnecessary controllers, missing disposal, and rebuilding entire trees

Type of Change

• New feature (feat)
• Bug fix (fix)
• Code refactor (refactor)
• Documentation (docs)
• CI change (ci)
• Chore (chore)