[BUG] Demo - the cleanup trap expands `$DEMO_TEMP_DIR` unquoted

[GingerGraham]

In this demo, the cleanup trap expands $DEMO_TEMP_DIR unquoted (trap 'rm -rf $DEMO_TEMP_DIR' EXIT). ShellCheck will flag this and it can misbehave if the path ever contains spaces/globs. Quote the variable inside the trap command.

trap 'rm -rf -- "$DEMO_TEMP_DIR"' EXIT

Originally posted by @Copilot in #56 (comment)