Add a non-main-session-writing sidechain to DeepSeek Harness using dsh-sidechain

The community plugin `dsh-sidechain` provides `/side` persistent side sessions and `/btw` one-off side questions for DeepSeek Harness, which run in temporary forks while the main session history remains unchanged. This article verifies against the plugin directory page and GitHub README: the maintainer is omdsh-dev, the version is 0.6.4, it is compatible with DSH rc.5 / rc.6, as well as the web sidechain panel, installation commands and isolation boundary.