ollie-9p computes its namespace path from $USER and $DISPLAY via
client.Namespace(). When run inside the sandboxed shell, USER was not
guaranteed to be in the environment, producing /tmp/ns..:0/ollie.
Fix: explicitly inject USER into envExtra in SetSessionEnv (alongside
OLLIE_SESSION_ID and OLLIE_UNAME). Also export the computed NAMESPACE
to cmd.Env so ollie-9p can use it directly without recomputing.