Add Ethora as open-source alternative to Sendbird, Stream, CometChat, Twilio Conversations

Following [CONTRIBUTING_TO_OSB.md](https://github.com/junaid33/opensource.builders/blob/main/CONTRIBUTING_TO_OSB.md) — submitting an open-source chat & messaging platform that's a direct alternative to several established proprietary tools.

**Tool to add:** [Ethora](https://github.com/dappros/ethora)
- Repository: https://github.com/dappros/ethora
- Website: https://ethora.com
- Live demo / playground: https://playground.chat.ethora.com
- License: Apache-2.0
- Stars: 530+
- Description: Open-source chat & messaging platform with SDKs (React, native iOS/Android, WordPress) and a built-in AI agent / chatbot framework. Self-host the server or use Ethora Cloud.
- Features: real-time chat (XMPP-based), 1:1 + group + rooms, message history, file uploads, push notifications, AI agents/chatbots with RAG, MCP server, web + mobile SDKs, self-hosted or hosted.

**Proprietary tools it's an alternative to:**

1. **Sendbird** — https://sendbird.com — Chat & messaging API platform
2. **Stream (getstream.io)** — https://getstream.io — Chat & activity feeds API
3. **CometChat** — https://cometchat.com — In-app messaging & calls SDK
4. **Twilio Conversations** — https://www.twilio.com/conversations — Messaging API

For each: similarity score ~80 (covers chat SDK, server, multi-platform clients, push, history, hosted + self-host options; Ethora additionally bundles an AI-agent / chatbot framework + an MCP server).

If any of those proprietary tools aren't yet on the site, please add them and link Ethora as an alternative. Happy to provide more detail on any feature mapping.