docs(gcp-backplane): record why GCP subjects are not scoped to a single BBD

.agents/references/gcp-backplane.md

@@ -90,6 +90,52 @@ The backplane's `credentials_json` output is then an
 document (`type = "external_account"`) pointing at the runner's token file, passed to the building
 block as a `FILE` input with `GOOGLE_APPLICATION_CREDENTIALS` set to its path.
 
+### Why GCP WIF subjects stop at the workspace
+
+`modules/azure/`, `modules/aws/` and `modules/stackit/` append the building block definition uuid to
+the subject; GCP deliberately does not because this creates a dependency cycle of the following form
+
+```hcl
+# --- backplane module ---
+resource "google_iam_workload_identity_pool_provider" "meshstack" {
+  # trust condition wants to name the specific BBD so that only a single BBD can assume the role
+  attribute_condition = "google.subject.startsWith('...buildingblockdefinition.${var.bbd_uuid}')"
+  ...
+}
+
+output "credentials_json" {
+  # audience must reference *this* pool provider's resource name
+  value = jsonencode({
+    audience = "//iam.googleapis.com/${google_iam_workload_identity_pool_provider.meshstack.name}"
+    ...
+  })
+}
+
+# --- root module ---
+resource "meshstack_building_block_definition" "gcp_storage_bucket" {
+  # the BBD needs the backplane's credentials as a static input...
+  spec = {
+    inputs = {
+      secret_value = "data:application/json;base64,${base64encode(module.backplane.credentials_json)}"
+    }
+  }
+}
+
+module "backplane" {
+  source = "./backplane"
+  # ...but the backplane needs the BBD's uuid to scope the trust condition, we now have a dependency cycle
+  bbd_uuid = meshstack_building_block_definition.gcp_storage_bucket.id
+}
+```
+
+The current workaround for this problem is to use an `attribute_condition` that does not pin the BBD uuid.
+A `startsWith` admits any building block definition owned by the same platform team workspace, so all of
+them share the backplane's federated identity — but authoring a definition in that workspace is
+already a privileged action, and in practice it coincides with being able to change the backplane
+itself. The residual cost is audit attribution: Cloud Audit Logs cannot tell which definition acted.
+Monitor https://feedback.meshcloud.io/feature-requests/p/introduce-building-block-definition-version-spec-resource-in-meshstack-terraform
+for progress on this matter.
+
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