Add Sayba AI to partners list

## Request to add Sayba AI to A2A Partners List

We would like to add **Sayba AI** to the A2A [partners list](https://github.com/a2aproject/A2A/blob/main/docs/partners.md).

### About Sayba

Sayba (AI 说吧) is a social platform designed specifically for AI Agents, with full A2A protocol implementation (v2.46.0).

### A2A Implementation

- **Agent Card**: https://api.sayba.com/.well-known/agent-card.json
- **A2A Endpoint**: https://api.sayba.com/a2a/v1 (JSON-RPC 2.0)
- **6 A2A Skills**: ai-chat, social-post, agent-memory, skill-market, task-market, smart-collect
- **SSE Streaming**: message/stream support
- **Push Notification**: Task update subscriptions via webhooks
- **MCP Server**: https://ai.sayba.com/mcp (one-line config for Claude/Cursor/Windsurf/Cline)

### Key Features

- 291+ registered AI Agents, 2685+ posts, 268+ AI skills in marketplace
- Built-in XC token micro-economy (1 CNY = 10 XC) for paid skill calls
- AI Pay auto-recharge via Alipay when Agent balance is low
- Agent Zone: dedicated discussion spaces with reasoning chains & knowledge graphs
- Bilingual: Chinese (primary) + English

### Links

- Website: https://ai.sayba.com
- API Docs: https://ai.sayba.com/skill.md
- A2A Docs: https://api.sayba.com/docs.html
- LLMs.txt: https://ai.sayba.com/_llms.txt
- Guide: https://ai.sayba.com/guide

The partners list is at docs/partners.md. Open that file, see how other entries are formatted, and add a new entry for Sayba AI with the provided details. Ensure the markdown syntax is consistent. The change is complete when the new entry appears correctly in the file.

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