Add skill: mapbox-offline-maps-patterns

Nobody has claimed this yet.

## Summary

Offline maps is a distinct and complex area of the Mapbox SDK with its own APIs for iOS and Android. AI assistants give generic or incorrect advice here because the offline pack and region download APIs are Mapbox-specific and not well-represented in training data.

## Topics to cover

- **iOS** - `OfflineManager`, `TileRegionLoadOptions`, tile region downloads, style pack downloads
- **Android** - `OfflineManager`, `TilesetDescriptorOptions`, offline region management
- Cache management - size limits, eviction, pre-warming
- Handling offline/online state transitions gracefully
- Conflict resolution between cached and live data
- Estimating download size before initiating

## Key patterns a skill should teach

- How to define a tile region with a geometry and zoom range
- Progress callbacks and error handling during download
- Checking whether a region is available before going offline
- Cleaning up stale offline regions

## Why this matters

Offline maps is a common enterprise and field-work requirement. Without a skill, AI assistants suggest patterns from old SDK versions or generic caching approaches that don't work with Mapbox's API.

Start by organizing the skill around Mapbox's iOS OfflineManager and TileRegionLoadOptions APIs, then cover the Android OfflineManager and TilesetDescriptorOptions APIs. Include tile-region and style-pack downloads, progress and error handling, cache management, offline/online transitions, download-size estimation, availability checks, and stale-region cleanup.

Written by the indexing model from the issue text.