cmux tiles panes not workspaces; custom sidebars bridge the gap

In cmux, the main view renders exactly ONE workspace at a time (compiled, no config/plugin/API hook exposes it), so separate workspaces can never tile as columns — only panes within one workspace tile. But custom sidebars are runtime-interpreted SwiftUI files in ~/.config/cmux/sidebars/ that hot-reload and bind to live state including per-surface tabs data (title, git branch/dirty, ports, focused). So one workspace split into N panes PLUS a custom sidebar iterating its tabs gives both simultaneous columns and per-column status rows. Caveat: rich signals (progress bar, latest agent message, PR status) exist only at workspace granularity, not per-surface.