Add Discord sink

[YuyangXueEd]

Summary

Add a Discord delivery sink so users can receive their daily digest in a Discord channel via an Incoming Webhook.

Why

Discord is widely used by developer and research communities. It uses the same incoming webhook pattern as the existing Slack sink, making this a well-scoped first contribution.

What to implement

A new sink at sinks/discord/ that:

• Sends the daily digest as one or more Discord messages using the Discord Incoming Webhook API
• Formats content using Discord's markdown (bold, links, code blocks)
• Reads the webhook URL from DISCORD_WEBHOOK_URL environment variable / GitHub Secret
• Respects the max_papers, max_hn, max_github config options (same as Slack sink)

How to get started

1. Copy sinks/_template/ to sinks/discord/
2. Look at sinks/slack/ as a reference implementation — the Discord webhook API is very similar
3. Add config under sinks: in config/sources.yaml:

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

4. Register in sinks/__init__.py
5. Add DISCORD_WEBHOOK_URL to .github/workflows/daily.yml env block (see how SLACK_WEBHOOK_URL is handled)
6. Write a test in tests/ — see tests/test_slack_sink.py for the pattern
7. Add a sinks/discord/README.md documenting setup

Acceptance criteria

• sinks/discord/__init__.py with DiscordSink class
• sinks/discord/README.md with setup instructions
• Registered in sinks/__init__.py
• Config block documented in config/sources.yaml (commented out, enabled: false)
• DISCORD_WEBHOOK_URL wired into daily.yml
• At least one test
• pytest tests/ -q passes