feat(examples): add useAskableSource and subscribeAsync examples — new APIs have no reference implementation

[vamgan]

Summary

The three newest public APIs — useAskableSource, subscribeAsync / toPromptContextAsync, and async sanitizeItem — have no working example in either examples/analytics-dashboard-react or examples/react-native-expo. Users adopting these features have no reference implementation to copy.

What's missing

1. useAskableSource / collection source

No example shows:

• Registering a collection source (e.g. a table or list of items)
• Calling handle.notifyChanged() when data updates
• Wiring the source into an AI sidebar

2. subscribeAsync / toPromptContextAsync

No example shows:

• Setting up an async subscriber for multi-turn context
• Passing async context into an LLM call (Vercel AI SDK, CopilotKit, Anthropic)
• Handling the onError callback

3. Async sanitizeItem

No example shows redacting or enriching individual collection items before they reach the AI.

Suggested additions

• Add a "data sources" panel to examples/analytics-dashboard-react that:
  • Registers a useAskableSource for the deals table
  • Subscribes with subscribeAsync and streams context into the chat sidebar
  • Uses sanitizeItem to strip an internalId field

Acceptance criteria

• At least one runnable example demonstrates useAskableSource + notifyChanged
• At least one example shows subscribeAsync wired into an LLM call
• Example includes inline comments explaining the async data flow