omdsh-basemode: DeepSeek Harness session mode registry, switcher, and sidebar dots

# Introduction The philosophy of DSH (DeepSeek Harness) is "everything is a plugin." When applied to the web GUI, this raises an immediate issue: both the Chat mode plugin and the Code mode plugin want to occupy the session bar. Who provides the "switch mode" toggle? In the sidebar, a row of sessions all look identical. Which ones belong to which mode, and which one are you currently viewing? If the registry is released with a specific mode, other modes would have to depend on that entire mode's package, inverting the layering relationship. The omdsh-basemode introduced below achieves exactly this by extracting this "seat" from the various "modes." # What is this `@omdsh-plugins/omdsh-basemode` (Plugin Center Display Name: Base Mode / 基