This only includes the core mechanisms to make it work. There's not yet
any way to turn this mode on as an end-user, because we have to do some
more work at the UI layer to present this well before we could include it
as an end-user-visible feature in a release.
At the lowest level of abstraction inside the graph nodes themselves, this
effectively mirrors the existing option to disable refreshing with a new
option to disable change-planning, so that either "half" of the process
can be disabled. As far as the nodes are concerned it would be possible
in principle to disable _both_, but the higher-level representation of
these modes prevents that combination from reaching Terraform Core in
practice, because we block using -refresh-only and -refresh=false at the
same time.
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