[Feature] Add Discord channel support

[Vitalcheffe]

Description

Add Discord as a channel option alongside Telegram. Users should be able to interact with their NEBULA agent through Discord servers.

Proposed implementation

• Create packages/channels/discord/ package
• Use discord.js v14
• Support slash commands: /start, /help, /status
• Support regular messages routed to core engine
• Rate limiting per user (same as Telegram)

Use case

Many developers use Discord as their primary communication platform. Having Discord support would make NEBULA accessible to a wider audience.

Priority

Medium — Telegram is working, Discord is a nice-to-have for v8.