Say that settings.yaml is shared only until the session's first settings write
(dsh saves by rename, which replaces the link), warn when a session's own DSH_HOME
does not answer and the server's home is linked instead, test
guardSessionConfigDirHome, and document the linkFromRealHome exception and what a
custom-model dsh session does not carry over.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>