Add Discord channel

[tuchg]

Goal

Add Discord as a Lucarne channel adapter.

Scope

• Map Lucarne workspaces to Discord server/channel/thread primitives.
• Implement inbound messages, buttons/interactions, file download/upload, and outbound formatting.
• Add configuration, onboarding/docs, and tests.

Acceptance criteria

• Users can run Lucarne through Discord with workspace-style conversations.
• Agent notifications, replies, approvals, and status commands work.
• Permission and token handling are documented.
• Channel-specific behavior stays in the Discord adapter.