v2.30.0: Smaller downloads, retries on a busy stack, stacks rm cleanup and private-key scrubbing

• Large inventories download much less. The skill now asks Meridian to compress its responses: a
  page of 100 assets shrank 28 times on a test stack. How much faster a question gets depends on how
  fast your stack itself answers.
• A busy stack slows an answer down instead of failing it. When Meridian says it is getting too many
  requests, the skill waits as long as it asks, up to a minute, says so, and tries again.
• Removing a stack stops using its credentials. Before, its token stayed in use until you switched.
  Another saved stack now takes over and the skill says which; its cached data is deleted, and
  --purge deletes its history too.
• Private keys are kept out of connector messages. An error that quoted an uploaded key file, such
  as a cloud service-account file, is now cleaned by recognising the key itself.