Add skill: mapbox-cross-platform-patterns (Flutter, React Native, Unity)

## Summary

AI assistants have no Mapbox-specific guidance for cross-platform mobile runtimes. Without a skill, they fall back on generic patterns that don't account for Mapbox SDK initialization, map lifecycle, or platform-specific idioms.

## Runtimes to cover

- **Flutter** - `mapbox_maps_flutter` plugin, widget lifecycle, camera, annotations
- **React Native** - `@rnmapbox/maps`, JSX map component patterns, gesture handling
- **Unity** - Mapbox Maps SDK for Unity, tile loading, AR integration

## Key patterns a skill should teach

- SDK initialization and access token configuration per runtime
- Map lifecycle (mount/unmount equivalents in each framework)
- Camera control and user interaction
- Adding markers and annotations
- Common mistakes specific to each runtime (e.g. not disposing the map in Flutter, not cleaning up in React Native)

## Why this matters

These are high-traffic runtimes with active developer communities. Base model advice for Mapbox on Flutter/React Native tends to be generic or outdated.

The issue does not name files, tests, or an entry point. First determine how skills are structured in this repository, then scope guidance for Flutter, React Native, and Unity around initialization, lifecycle, camera interaction, annotations or markers, and runtime-specific cleanup. Done means the skill covers the listed runtimes and key patterns without relying on generic Mapbox advice.

Written by the indexing model from the issue text.