1.  Don't describe the `.use-civicrm-setup` protocol.  This is used in one
    place, and it's likely to get inverted in the near future.

2.  When customizing the model, most values shouldn't be passed directly
    into `init()`.  Let `init()` detect its defaults.  Then call
    `$setup->getModel()` and customize that.  (I had tried customizing via
    `init()` when first implementing `cv core:install` and found it less
    useful.  There was also a support issue when someone else tried the same
    thing.) This pattern should work more intuitively.