New stack seahaven-terraform-substrate (instances terraform-substrate-prod +
terraform-substrate-dev): app.terraform.io OIDC provider and the shared
boundary-gated guardrail policy seahaven-hcptf-iam-management that
per-workspace Terraform apply roles attach at migration time. No roles are
pre-provisioned (accumulator pattern, parallel to githubdeploy-*).
Guardrail statements mirror seahaven-cfn-exec-iam-management byte-identically
except DenySelfMutation, whose scope extends to hcptf-* alongside the
GitHub-substrate principals. Explicit stack dependency on the same-account
deploy-substrate stack (boundary ARN appears only in Condition strings, so
CFN infers no edge).
