Add Telegram sink

[YuyangXueEd]

Summary

Add a Telegram delivery sink so users can receive their daily digest as a Telegram message via the Bot API.

Why

Telegram is a primary notification channel for many users, especially in China and Europe. It is also popular in academic and open-source communities.

What to implement

A new sink at sinks/telegram/ that:

• Sends the daily digest using the Telegram Bot API (sendMessage endpoint)
• Formats content in Telegram's MarkdownV2 or HTML parse mode
• Reads credentials from environment variables:
  • TELEGRAM_BOT_TOKEN — bot token from @Botfather
  • TELEGRAM_CHAT_ID — target channel or chat ID
• Supports splitting long digests into multiple messages if needed (Telegram has a 4096 character limit per message)
• Respects max_papers, max_hn, max_github config options

How to get started

1. Copy sinks/_template/ to sinks/telegram/
2. Look at sinks/slack/ and sinks/serverchan/ as reference implementations
3. Add config under sinks: in config/sources.yaml:

   sinks:
     telegram:
       enabled: false
       max_papers: 5
       max_hn: 3
       max_github: 3

4. Register in sinks/__init__.py
5. Add TELEGRAM_BOT_TOKEN and TELEGRAM_CHAT_ID to .github/workflows/daily.yml env block
6. Write a test in tests/ — see tests/test_slack_sink.py for the pattern
7. Add a sinks/telegram/README.md documenting how to create a bot and get a chat ID

Acceptance criteria

• sinks/telegram/__init__.py with TelegramSink class
• sinks/telegram/README.md with setup instructions (bot creation, chat ID lookup)
• Registered in sinks/__init__.py
• Config block documented in config/sources.yaml (commented out, enabled: false)
• Both secrets wired into daily.yml
• At least one test
• pytest tests/ -q passes