Add Discord channel support

[owainlewis]

Goal

Support Discord as a next messaging channel after iMessage and Telegram are stable.

Why

Discord is useful for team and community workflows, but it should not block the core personal gateway replacement.

Scope

• Configure a Discord bot token and allowlisted users, channels, or servers.
• Receive DMs or selected channel messages.
• Send replies back to the originating conversation.
• Map Discord conversations to stable push thread ids.

Acceptance criteria

• Allowlisted Discord messages can route to Claude Code or Codex.
• Non-allowlisted Discord messages are ignored.
• Discord state is isolated from iMessage and Telegram state.
• Channel-specific setup and permissions are documented.