91c0feaa87
The setup window between writeIsolationOwner and isoStart left the base dir holding only the marker file and no `m` mount, so classifyDir returned null and scanWorktrees classified it as a stray — which a non-`--all` clear removes, defeating the ownership guard mid-setup. classifyDir now treats the presence of the ownership marker as a task-isolation signal (in addition to the mount dir), so an in-progress sandbox with a live owner is preserved throughout backend setup. Fixes #6761