Add README for Agent Stack Client SDK (#1671)

This README provides an overview of the Agent Stack Client SDK, including installation instructions, key features, and examples of usage.

Signed-off-by: Jenna Winkler <[email protected]>

Author: jenna-winkler

apps/agentstack-sdk-ts/README.md

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+# Agent Stack Client SDK
+
+TypeScript/JavaScript client SDK for building applications that interact with Agent Stack agents.
+
+[![npm version](https://img.shields.io/npm/v/agentstack-sdk.svg?style=plastic)](https://www.npmjs.com/package/agentstack-sdk)
+[![License](https://img.shields.io/badge/License-Apache_2.0-blue.svg?style=plastic)](https://opensource.org/licenses/Apache-2.0)
+[![LF AI & Data](https://img.shields.io/badge/LF%20AI%20%26%20Data-0072C6?style=plastic&logo=linuxfoundation&logoColor=white)](https://lfaidata.foundation/projects/)
+
+## Overview
+
+The `agentstack-sdk` provides TypeScript/JavaScript tools for building client applications that communicate with agents deployed on Agent Stack. It includes utilities for handling the A2A (Agent-to-Agent) protocol, managing agent extensions, and working with the Agent Stack platform API.
+
+## Key Features
+
+- **A2A Protocol Support** - Full support for Agent-to-Agent communication
+- **Extension System** - Built-in handlers for forms, OAuth, LLM services, MCP, and more
+- **Platform API Client** - Utilities for context and token management
+- **TypeScript Types** - Comprehensive types for all APIs
+
+## Installation
+
+```bash
+npm install agentstack-sdk
+```
+
+## Quickstart
+
+```typescript
+import { handleAgentCard, handleTaskStatusUpdate, TaskStatusUpdateType } from 'agentstack-sdk';
+
+// Parse agent capabilities
+const { extensions, fulfillments } = await handleAgentCard(agentCard);
+
+// Send message and handle responses
+const stream = client.sendMessage(message);
+
+for await (const event of stream) {
+  const result = handleTaskStatusUpdate(event);
+  
+  switch (result.type) {
+    case TaskStatusUpdateType.Message:
+      console.log('Agent response:', result.message.parts[0].text);
+      break;
+    case TaskStatusUpdateType.InputRequired:
+      // Handle extension demands (forms, OAuth, etc.)
+      break;
+  }
+}
+```
+
+## Available Extensions
+
+The SDK includes clients and specs for handling:
+
+- **Forms** - Static and dynamic form handling (`FormExtensionClient`, `FormExtensionSpec`)
+- **OAuth** - Authentication flows (`OAuthExtensionClient`, `OAuthExtensionSpec`)
+- **LLM Services** - Model access and credentials (`LLMServiceExtensionClient`, `buildLLMExtensionFulfillmentResolver`)
+- **Platform API** - Context and resource access (`PlatformApiExtensionClient`, `buildApiClient`)
+- **MCP** - Model Context Protocol integration (`MCPServiceExtensionClient`)
+- **Embeddings** - Vector embedding services (`EmbeddingServiceExtensionClient`)
+- **Secrets** - Secure credential management (`SecretsExtensionClient`)
+- **Citations** - Source attribution (`citationExtension`)
+- **Agent Details** - Metadata and UI enhancements (`AgentDetailExtensionSpec`)
+
+Each extension has a corresponding `ExtensionClient` for sending data and `ExtensionSpec` for parsing agent cards.
+
+## Resources
+
+- [Agent Stack Documentation](https://agentstack.beeai.dev)
+- [GitHub Repository](https://github.com/i-am-bee/agentstack)
+- [npm Package](https://www.npmjs.com/package/agentstack-sdk)
+
+## Contributing
+
+Contributions are welcome! Please see the [Contributing Guide](https://github.com/i-am-bee/agentstack/blob/main/CONTRIBUTING.md) for details.
+
+## Support
+
+- [GitHub Issues](https://github.com/i-am-bee/agentstack/issues)
+- [GitHub Discussions](https://github.com/i-am-bee/agentstack/discussions)
+
+---
+
+Developed by contributors to the BeeAI project, this initiative is part of the [Linux Foundation AI & Data program](https://lfaidata.foundation/projects/). Its development follows open, collaborative, and community-driven practices.