Delete a campaign

Permanently delete a campaign. Its outreach stops for good, so no further steps are sent: remaining steps are skipped and queued emails cancelled (messages and invitations already sent are not retracted). The campaign's lists and contacts are kept. Its outreach logs (GET /api/v1/campaigns/{id}/logs) are no longer retrievable through the API. Deletion cannot be undone. An active campaign can be deleted directly; it does not need to be deactivated first. A campaign is the outreach side of an agent, so this also deletes the lead source agent with the same id. Needs the campaigns:write scope.