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>

mgsh_test.go

@@ -943,3 +943,45 @@ func TestExpandAlias(t *testing.T) {
 		}
 	}
 }
+
+func TestSplitAtMarker(t *testing.T) {
+	lines := []string{"a", "b", "---mgsh---", "c", "d"}
+	before, after := splitAtMarker(lines, "---mgsh---")
+	if strings.Join(before, ",") != "a,b" || strings.Join(after, ",") != "c,d" {
+		t.Errorf("split = %v / %v", before, after)
+	}
+	// no marker: everything is the first section, so a server that produced no
+	// du output simply yields no sizes
+	before, after = splitAtMarker([]string{"a", "b"}, "---mgsh---")
+	if strings.Join(before, ",") != "a,b" || after != nil {
+		t.Errorf("split without marker = %v / %v", before, after)
+	}
+}
+
+func TestParseDuSizes(t *testing.T) {
+	lines := []string{
+		"185432\tBetaflight3.0.0.git",
+		"2144\twebsite.git",
+		"876   spaced-with-blanks.git", // some du implementations use spaces
+		"1024\t./with-dot-slash.git",
+		"1500\tmy project.git", // a name with a space survives
+		"garbage",
+		"",
+	}
+	got := parseDuSizes(lines)
+	want := map[string]int64{
+		"Betaflight3.0.0.git":    185432 * 1024,
+		"website.git":            2144 * 1024,
+		"spaced-with-blanks.git": 876 * 1024,
+		"with-dot-slash.git":     1024 * 1024,
+		"my project.git":         1500 * 1024,
+	}
+	if len(got) != len(want) {
+		t.Fatalf("parseDuSizes = %v, want %d entries", got, len(want))
+	}
+	for k, v := range want {
+		if got[k] != v {
+			t.Errorf("parseDuSizes[%q] = %d, want %d", k, got[k], v)
+		}
+	}
+}