The Docker refusal on POST /api/sessions/:id/folders tested only `~/`, but
listFolder also expands a bare `~` to this host's home, so a Docker case
could list and serve the host home through it (the preview's "Show folder"
for a `~/x.png` link reached it with no typing). A bare `~` now gets the same
400.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>