Add Expo plugin (#139)

## Summary
- Add the Expo plugin with 13 skills for Expo Router UI, API routes,
data fetching, native modules, dev clients, deployment, workflows, and
SDK upgrades
- Wire Expo metadata, logo asset, marketplace registration, README
listing, and authenticated Expo MCP endpoint
- Add `codex-expo-run-actions` guidance for `script/build_and_run.sh`
and Codex Run/action-button setup

## Testing
- Verified Expo plugin manifest with validator: `skills: 13`, `mcp: 1`
- Ran JSON parse checks for plugin / marketplace / MCP metadata
- Ran `node --check` for Expo workflow helper scripts
- Ran `git diff --check`
- Verified public Expo metadata links and MCP endpoint response

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Co-authored-by: Kudo Chien <ckchien@gmail.com>

README.md

@@ -14,4 +14,5 @@ Highlighted richer examples in this repo include:
 - `plugins/build-ios-apps` for SwiftUI implementation, refactors, performance, and debugging
 - `plugins/build-macos-apps` for macOS SwiftUI/AppKit workflows, build/run/debug loops, and packaging guidance
 - `plugins/build-web-apps` for deployment, UI, payments, and database workflows
+- `plugins/expo` for Expo and React Native apps, SDK upgrades, EAS workflows, and Codex Run actions
 - `plugins/netlify`, `plugins/remotion`, and `plugins/google-slides` for additional public skill- and MCP-backed plugin bundles