infra: document and enforce the Azure isolation boundary

Antifailure creates resources only in af-dev-scus, af-corpus-scus, af-cp-scus,
and af-tfstate-scus, all tagged project=antifailure. Nothing outside those four
groups is created, modified, read, or deleted, and no role is ever assigned at
subscription scope.

Records the resource groups that already exist in this subscription so the
guard has something concrete to protect, and describes the three independent
enforcement points: per group Terraform workspaces, identity scoped to the
three working groups only, and tools/azguard, which refuses any operation whose
target group does not start with af- and carry the project tag.

Signed-off-by: Vir Sanghavi <Virrsanghavi@gmail.com>