Organize backend into product executable and shared Go modules

backend/AGENTS.md

@@ -25,11 +25,11 @@ skill to perform this completion workflow.
 ## Use the project error model
 
 - Do not introduce `fmt.Errorf` or `errors.New`. Use the constructors and
-  wrapping helpers in `backend/pkg/merrors`.
+  wrapping helpers in `backend/shared/errors`.
 - Preserve the underlying cause when adding context by using an appropriate
   `merrors` wrapping helper.
 - When no suitable project error exists, add the smallest generally useful
-  definition or helper to `backend/pkg/merrors` instead of creating an ad hoc error
+  definition or helper to `backend/shared/errors` instead of creating an ad hoc error
   elsewhere.
 
 ## Keep modules cohesive and names concise
@@ -93,7 +93,18 @@ unused dependencies; remove those dependencies from the selected components.
 Copied BFF components are locally owned and may diverge.
 Use only Sendico's public HTTP API for platform operations. Never import sibling
 Sendico modules or access its database, internal gRPC APIs, discovery or message
-bus. Keep the pinned public wire contract separate from local DTOs and models.
+bus. Keep consumed public wire contracts inside the owning adapter, separate
+from local DTOs and models. Do not copy unused API snapshots.
 Resolve the configured provider organization and customer resource ownership on
 the server. Never treat all customers as the backend integration principal.
 Use the copied structured logger, injected at operational boundaries.
+
+## Go module structure
+
+`backend/` groups independent Go modules; it is not itself a Go module.
+`product/` is the customer-facing executable and owns its `main.go` and `go.mod`.
+`shared/` is a library module for reusable infrastructure and has no entry point.
+Backoffice belongs in its own executable module (`backoffice/`) when implemented,
+with its own entry point and configuration, not inside the product executable.
+Keep feature models, storage, services and transport together inside their owning
+module. Shared infrastructure must not import a product or backoffice module.