[idea] Workspace-attached right plugin panes

[paulbettner]

idea / problem

Plugin panes are currently tied to a tab layout or transient overlay/popup. A project explorer, review view, or messaging view often belongs to the workspace: it should stay visible while switching tabs, without being added to every tab's split tree.

requested change

Allow a plugin terminal pane to attach to a workspace's right edge. Each workspace should independently remember whether the pane is attached and focused; switching tabs keeps it present, and switching workspaces restores that workspace's own state.

This differs from #633 (moving the built-in agent panel) and #1675 (global plugin-contributed sidebar areas): the surface is optional and workspace-scoped.

why you want this

It provides a stable home for project-scoped tools such as file explorers and code-review panes without duplicating them across tabs or disturbing tab layouts.