Add registerEvent to the React Native SDK

[V3RON]

Part of #95. Needs #126 and #127.

Why

React Native apps post events that agents can't discover, and app authors get no feedback when a posted event doesn't match what they meant to post.

Expected outcome

registerEvent({ name, description, payloadSchema? }) in @appduct/react-native, returning what registerTool returns. It goes through new TurboModule methods registerEvent(json) and unregisterEvent(name) to the native cores. payloadSchema takes a Standard Schema or raw JSON Schema, as tool schemas do. The JS layer keeps the Standard Schemas and uses them for development warnings.

Constraints and non-goals

• Declaring an event is advisory. An undeclared postEvent still reaches agents, and every existing postEvent caller keeps working unchanged.
• Production builds post as-is, with no checks.

How we know it is done

1. After registerEvent({ name: "checkout_completed", description, payloadSchema }), events.list returns the event with its description and schema, and after the disposer runs it's gone.
2. In development, posting an undeclared name warns and names the event, and the event still reaches appduct_events.
3. In development, posting a payload that fails the declared schema warns.
4. In production, neither warning fires, and payloads are posted as-is.
5. docs/TOOLS.md and the shipped skill's writing-tools.md tell app authors to declare each event they post.

Alternatives considered

none