[Feature]: Flutter AG-UI client — agentivity_ag_ui (pub.dev)

### Pre-flight Checklist

- [x] I have searched existing issues and this hasn't been requested yet.

### Problem or Motivation

Hi 👋

I've built a Flutter implementation of the AG-UI protocol: **agentivity_ag_ui**.

https://pub.dev/packages/agentivity_ag_ui
https://github.com/agentivity-labs/agentivity_ag_ui

## Request

Could you add `agentivity_ag_ui` to the **Clients** table in the README?
Flutter is currently the only mobile-native AG-UI client — it fills the same role
as CopilotKit does for React, but for iOS/Android/Web Flutter apps.

The exact row would be:

| [Flutter — agentivity_ag_ui](https://github.com/agentivity-labs/agentivity_ag_ui) | ✅ Supported | ➡️ [Getting Started](https://github.com/agentivity-labs/agentivity_ag_ui#get-started-in-your-own-app) · [pub.dev](https://pub.dev/packages/agentivity_ag_ui) | Community |

## What it covers

- SSE connection + AG-UI event parsing
- Streaming text messages (token-level)
- Tool calls with status tracking
- Generative UI via `AgUiWidgetRegistry` (agent renders named Flutter widgets)
- State sync via RFC 6902 JSON Patch
- Human-in-the-loop — interrupt + resume flow
- Reasoning blocks (collapsible chain-of-thought)

## Ready-made connectors

- `AgentivityConnector` — Agentivity platform
- `LangGraphConnector` — LangGraph Cloud / self-hosted
- `AgUiGenericConnector` — any AG-UI SSE endpoint

## Works with

iOS · Android · Web · Windows — compatible with Riverpod, Provider, BLoC, GetX.

Happy to be added to the Clients table in the README if that fits.

### Proposed Solution

https://pub.dev/packages/agentivity_ag_ui

### Alternatives Considered

_No response_

### Additional Context

_No response_