listGroupDirectMessages lists multi-person direct-message conversations. Use
it when you need Slack’s group-DM conversation IDs rather than channel IDs.
Example
automations/list-group-dms.automation.ts
Inputs
Output
ReturnsSignal<{ conversations: SlackConversation[]; responseMetadata: { nextCursor?: string } }>.
Group DMs report isGroupDirectMessage when Slack supplies the flag. Continue
pagination with nextCursor.