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>

README.md

@@ -134,14 +134,20 @@ worked-on project sits closest to the prompt:
 
 ```
 < src > list
-  Betaflight3.0.0  Sep 28  2016
-  website          Mar  3  2024
-  notes            Jan  3 14:32
-3 repositories
+  Betaflight3.0.0  Sep 28  2016     181M
+  website          Mar  3  2024     2.1M
+  notes            Jan  3 14:32     876K
+3 repositories · 184M
 ```
 
-`list -a` lists the archives instead, with their sizes; a pattern filters by
-name (`list note`).
+The size is the repository's real disk usage on the server, asked of `du` in
+the same round trip as the listing — a long listing reports the inode size for
+a directory, which is the same number for every repository and says nothing. If
+the server produces no usable sizes the column is left out rather than filled
+with zeroes.
+
+`list -a` lists the archives instead, whose sizes come from the listing itself;
+a pattern filters by name (`list note`).
 
 ### Overview