After task.finish removed a worktree, the task's workspace stayed in the
list with a cwd that no longer existed. Once the checkout is gone, the
workspaces rooted at it now close with their panes, on every finish
path (discard, merge and the cleanup retry). A checkout that survives
keeps its workspace.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>