Add Discord bot with Gateway WebSocket forwarding

[RSO]

Summary

• Implements the Discord bot with a Gateway WebSocket forwarding pattern (inspired by vercel/chat)
• Bot receives @mentions via Gateway, processes them through the platform-agnostic runBot() engine, and replies with AI responses
• Includes spawn_cloud_agent tool support for coding tasks on GitHub repos

Depends on: #558

Architecture: Gateway-to-Webhook Forwarding

Discord Gateway (WebSocket via discord.js)
    |
    v
/api/discord/gateway  (cron-triggered, runs ~4 min)
    |  forwards MESSAGE_CREATE via HTTP with x-discord-gateway-token
    v
/discord/webhook  (unified handler)
    |  also handles PING verification + slash commands via Ed25519
    v
discord-bot.ts -> runBot() -> spawn_cloud_agent

Why Gateway is needed: Discord's HTTP Interactions API only delivers slash commands and component interactions -- not regular chat messages. To receive @bot mentions in channels, a Gateway WebSocket connection is required.

How it works:

1. A cron job hits /api/discord/gateway every ~3 min
2. It connects to Discord Gateway via discord.js, listens for raw events
3. MESSAGE_CREATE events are forwarded as HTTP POST to /discord/webhook with x-discord-gateway-token header for auth
4. The webhook handler processes the message: strips bot mention, resolves user names, adds hourglass reaction, runs the AI bot, posts reply, swaps to checkmark reaction

New files

File	Purpose
src/app/api/discord/gateway/route.ts	Cron-triggered Gateway WebSocket listener
src/app/discord/webhook/route.ts	Unified webhook handler (interactions + forwarded events)
src/lib/discord-bot.ts	Main bot processor (reuses runBot() engine)
src/lib/discord/auth.ts	Auth token generation for bot user
src/lib/discord/verify-request.ts	Ed25519 signature verification
src/lib/discord-bot/discord-utils.ts	Mention stripping, name resolution, truncation
src/lib/discord-bot/discord-channel-context.ts	Channel info + recent messages for AI context

Modified files

File	Change
src/lib/integrations/discord-service.ts	Added postDiscordMessage, addDiscordReaction, removeDiscordReaction
src/lib/bot-users/types.ts	Added 'discord-bot' to BotType
src/lib/feature-detection.ts	Added 'discord' to FEATURE_VALUES
src/lib/config.server.ts	Added DISCORD_PUBLIC_KEY env var

New dependencies

discord.js, discord-interactions, discord-api-types

New env var

DISCORD_PUBLIC_KEY -- from the Discord Developer Portal, used for Ed25519 verification of HTTP Interactions

Cron setup (for Gateway)

{ "path": "/api/discord/gateway?cron_secret=...", "schedule": "*/3 * * * *" }