feat(plugins): add Telegram channel plugin

plugins/telegram/README.md

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+# @mosaic/telegram-plugin
+
+`@mosaic/telegram-plugin` connects a Telegram bot to the Mosaic gateway chat namespace so Telegram chats can participate in the same conversation flow as the web, TUI, and Discord channels.
+
+## Required Environment Variables
+
+- `TELEGRAM_BOT_TOKEN`: Bot token issued by BotFather
+- `TELEGRAM_GATEWAY_URL`: Base URL for the Mosaic gateway, for example `http://localhost:3000`
+
+## What It Does
+
+- Launches a Telegram bot with `telegraf`
+- Connects to `${TELEGRAM_GATEWAY_URL}/chat` with `socket.io-client`
+- Maps Telegram `chat.id` values to Mosaic `conversationId` values
+- Forwards inbound Telegram text messages to the gateway as user messages
+- Buffers `agent:start` / `agent:text` / `agent:end` socket events and sends the completed response back to the Telegram chat
+
+## Getting a Bot Token
+
+1. Open Telegram and start a chat with `@BotFather`
+2. Run `/newbot`
+3. Follow the prompts to name the bot and choose a username
+4. Copy the generated token and assign it to `TELEGRAM_BOT_TOKEN`