Notification Policies Access to Variables

We use stack contexts and environment variables to store useful metadata, such as the version being deployed.

However, notification policies do not have access to those context variables. Because of this, we have to run custom scripts before the plan step to expose values as flags, just so notifications can read them.

It would be much cleaner if notification policies had access to the same variables that plan policies do, including context-attached environment variables.

This would remove the need for workarounds and simplify our notification logic.