Show repository sizes in list

`list -a` had sizes because archives are files; repositories are
directories, and a long listing reports the inode size for those -- 4096
for every single one. Taking that number would have filled the column
with the same meaningless value, so the real disk usage is asked of `du`
instead, appended to the same remote command so it still costs one round
trip.

The column is dropped entirely when no usable sizes come back, rather
than showing a column of zeroes, so a server without a working `du`
degrades to the previous output. The summary line carries the total.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

version.txt

@@ -1 +1 @@
-4.0.29
+4.0.31