feat: add React Native template

[jerelvelarde]

Adds a CopilotKit React Native starter as Template 3 alongside Slack and web. The Expo app shares phone context with the agent, renders account/budget/spending/activity cards, and lets the user approve an expense that updates local sample data.

The isolated /api/mobile-copilotkit runtime uses the existing server-side OpenAI/OpenRouter configuration. A small shared-agent option selects the mobile prompt and excludes workplace MCP tools for this runtime while preserving existing defaults. Provider keys stay on the server.

Expense validation protects both the approval UI and the mutation helper. Invalid or incomplete proposals, unknown accounts and mismatched currencies are rejected visibly. Summaries preserve currencies, and successive transactions receive distinct IDs. Chat uses the CopilotKit facade for tools/context, supports Hermes-safe message IDs, shows runtime errors, and renders assistant Markdown with native styles and HTTP(S) links. A narrow Metro resolver adjustment supports the transitive browser-compatible jose package.

An animated sequence of reviewed screenshots from the live iOS/OpenAI walkthrough.

Validation

• All three GitHub CI jobs passed on final head 7e3622e.

• Clean root/mobile installs, workspace/mobile TypeScript checks, root verification, 12 native regression tests, iOS/Android Metro exports and the web production build passed.

• Runtime, finance, chat and Markdown source reviews passed after fixes. Workflow lint and changed Markdown links checked; all relative links pass. The inherited Exa dashboard URL returned429. No package formatter/lint script is configured. The build retains the existing Google Vertex dynamic dependency warning.

• Real iOS Simulator + OpenAI gpt-5.6-sol validation: native account cards, explicit expense approval and cancellation, formatted Markdown headings/lists/code, and a link that opens Safari. Walkthrough captures document the final app.

• Expenses change in-memory sample data only. Live OpenRouter, physical-device networking and OCR were not verified.

Integration

Targets main after merged #13 — Remove the WhatsApp example. Preserves main's simplified web UI and media-tooling cleanup; PR #7 is not required. The revised homepage and guides present Slack, web and React Native. Broader homepage/folder/CTA cleanup and CI removal are tracked separately in the roadmap.

Inspired by CopilotKit/CopilotKit#5430, examples/showcases/react-native-personal-finance at 6815a3eed0d80570cc17c121d952b94d5543d0a7, adapted to this kit's Expo app and shared runtime.