Automate branch cutting for releases

[deansheather]

Automate code freeze and release branch cutting. This ensures that dogfood starts tracking the release branch without human intervention, so the release gets the full freeze period of dogfooding regardless of PTO or human frailty.

1. On the agreed day, an automated action
   1. tags the cut commit
   2. creates the release branch
   3. posts to #dev channel in Slack that we are frozen

relates to #1021