Add Interactive Long-Session Background Agents to DeepSeek Harness with dsh-background-agents

`dsh-background-agents` is a DSH session and messaging plugin maintained by PerryLink (Apache-2.0 license, npm version 0.5.1). It launches resumable background sub-agents on official sub-agent seams, providing five tools including `background_agent`, `bg_message`, `bg_list`, `bg_result`, `bg_stop`, as well as a web sidebar panel. Starting from version v0.5.0, team rooms are also optional. Install via the catalog page: `dsh plugin add github:PerryLink/dsh-background-agents`. It is compatible with DSH 0.1.0-rc.6. The plugin has the same permissions as the current dsh process, so you should check the source code and license before installation.