cmd_restart only persisted channel + chat_id across the os.execv boundary, so
when the new process announced "Restart completed" the OutboundMessage had
no Slack thread_ts and the reply fell back to the channel root.
Serialize msg.metadata into NANOBOT_RESTART_NOTIFY_METADATA, restore it on the
RestartNotice, and forward it to OutboundMessage so the completion message
follows the same routing as the original /restart invocation.
Made-with: Cursor
