add fallback deploy branch values

Updates job rules to compare against an environment-dependent default branch when deploy_branch is empty. For dev, this is CI_DEFAULT_BRANCH, and for uat/prod, this is the corresponding string defined in the policies for the 5065 Promotion Framework (release/5065-<env>).

Removes auto_start_dev input, replacing with the more generalized start_when. Use case: Merging to a promotion branch in a repo where deployment is just an ECR push.