Self-archive

Only act on an explicit request to archive this thread. Never use bb thread delete.

1. Send a brief closing summary before running commands.
2. Archive this thread and its children. If this fails, report the error and do not stop the runtime.

   bb thread archive --self --json
   

3. After archiving succeeds, stop the runtime. Run nothing afterward.

   bb thread stop --self --json
   

Undo: bb thread unarchive <id> --json.