feat: define clean npm package API for RN agent

[jamilagibson]

Context

The RN agent currently bootstraps inside the demo app. For npm distribution, developers need a single clean import with a defined API surface.

Target DX

import { initReactimeNative } from 'reactime-native';

// In app entry point:
initReactimeNative({ port: 8080 }); // optional config

Acceptance criteria

• package.json has correct main, types, and exports fields
• Peer dependencies declared: react, react-native, redux
• initReactimeNative is the single public entry point
• All public API types exported for TypeScript consumers
• Package installable via npm install reactime-native --save-dev in a fresh RN project