Feature: Add Discord channel support

[sesam]

Summary

Add Discord as a new channel alongside the existing Telegram and Slack integrations. The current architecture already anticipates Discord — types.ts references Discord in both the acknowledge method comment (line 19: "Discord: typing indicator") and the Destination interface (line 52: "Discord thread"), so this would be a natural next step.

Motivation

Discord is one of the most widely used community/team platforms, especially in developer and AI communities. Adding it as a channel would significantly expand Utah's reach and make it usable in many more contexts.

Proposed implementation

The channel-agnostic design of the agent loop, reply dispatch, and acknowledgment means no changes are needed outside src/channels/ (except registering the handler in index.ts).

New files to create in src/channels/discord/

File	Purpose	Complexity
api.ts	Discord REST API client — send messages (POST /channels/{id}/messages) and typing indicators (POST /channels/{id}/typing). Could use @discordjs/rest or raw fetch.	Low
handler.ts	Implements ChannelHandler with sendReply, acknowledge, and setup	Low
transform.ts	Maps Discord webhook payloads to AgentMessageData — e.g. d.channel_id → chatId, d.author.id → senderId, d.content → text	Medium
setup.ts	Webhook/bot registration + Ed25519 signature verification for incoming requests	Medium–High
format.ts	Markdown conversion — mostly pass-through since Discord supports standard MD, but needs 2000-char message chunking	Low

File to modify

• src/channels/index.ts — register the new DiscordHandler in the channel registry

Environment variables

DISCORD_BOT_TOKEN=...
DISCORD_APPLICATION_ID=...
DISCORD_PUBLIC_KEY=...   # for Ed25519 webhook signature verification

Discord-specific considerations

1. Signature verification (Ed25519): Discord requires verifying X-Signature-Ed25519 and X-Signature-Timestamp headers on every incoming webhook request. This is stricter than Telegram and should be handled in the transform or a middleware layer.

2. Interaction acknowledgment: Discord's Interactions Endpoint requires responding to PING events with an immediate PONG, which needs handling in the transform or an edge function.

3. Message length limit: Discord caps messages at 2000 characters, so format.ts needs chunking logic for longer agent replies.

4. Bot vs. Webhook approach: A bot-based approach (connecting to the Gateway) is more flexible for receiving messages than a pure webhook setup. Worth deciding which path to take.

Estimated effort

For a TypeScript dev familiar with Discord's API, roughly half a day to a full day of work. The hardest part is the Ed25519 signature verification and interaction acknowledgment — the rest maps cleanly onto the existing channel abstractions.

Happy to take a stab at this if the team is open to contributions!