v1.8.13 — list tools agents could not read

v1.8.13 — list tools that agents could not read, and a documented key the code ignores

• Three list tools answered in a shape the family contract does not use.
  list_dvs_portgroups returned {total, returned, portgroups},
  list_host_vmks returned {total, returned, vmks}, and
  scan_datastore_images returned {images, last_scan} with no count at all.
  Each is self-consistent; each is wrong, because an agent that has learned this
  family reads items and gets nothing. The absent keys are the ones that
  carry meaning: truncated and hint exist because of issue #31, where a
  model handed a bare list "incorrectly states that no data was returned". All
  three now return the envelope, with the old keys kept as deprecated aliases so
  nothing that reads them breaks.
• Offset paging claimed there was more at the end of the list. Truncation
  was derived from returned < total, which is wrong once offset is
  involved — the last page of five rows fetched with offset=4 returns one and
  reported truncated: true, advising the caller to raise a limit that cannot
  help them. Both offset-supporting tools now share one helper.
• The SSL config key in every doc was one the code has never read. Docs said
  disableSslCertValidation: true; config.py reads verify_ssl. Following the
  instruction produced a config that is ignored.

Found by running against a real vCenter rather than a mock.