List connected Slack workspaces

The account's active Slack workspaces with the ids a monitor needs for Slack delivery: create or PATCH a monitor with notify_slack: true, slack_integration_id set to an id here, and optionally slack_channel_id (default_channel_id when omitted). Slack is connected in the dashboard, never through the API; an empty list means the user must connect it there first. Reads stored data only. Single page, no pagination.