* fix(app): only surface failed workspace setup
Running and successful setup snapshots were treated as reasons to seed a background Setup tab, adding noise to every workspace with setup commands. Seed only failed setup and classify that tab as main-workspace content so the failure stays visible without stealing focus.
* fix(app): surface failed setup once
Keep the failed-setup surfacing claim with the setup snapshot so closing its background tab remains respected after the workspace remounts. Explicitly prefer the main pane for a new failure tab without moving a user-placed existing tab.
* fix(app): keep setup tabs out of side panel
A preferred main-pane placement can fall back when users remove that pane. Keep that implicit fallback out of the focused Side panel when another workspace pane is available.
