docs: declare institutional architecture boundary

AGENTS.md

@@ -1,5 +1,11 @@
 # GovOPlaN Postbox Codex Guide
 
+## Documentation Contract
+
+- Treat documentation as part of every behavior change. Update this module's manifest-driven `DocumentationTopic` contributions for affected user and administrator behavior.
+- Keep feature content here; `govoplan-docs` projects it without importing Postbox internals.
+- Maintain a static user/admin baseline and run `/mnt/DATA/git/govoplan/tools/checks/check-manifest-shapes.py` after behavior or manifest changes.
+
 ## Scope
 
 This repository owns the `postbox` module: in-platform postboxes, function-organization-bound access, postbox messages, postbox directory APIs, internal and portal postbox surfaces, campaign postbox integration, postbox-owned migrations, and future `@govoplan/postbox-webui`.

src/govoplan_postbox/backend/manifest.py

@@ -28,6 +28,7 @@ from govoplan_core.core.modules import (
     PermissionDefinition,
     RoleTemplate,
 )
+from govoplan_core.core.provider_governance import declared_module_architecture
 from govoplan_core.core.notifications import CAPABILITY_NOTIFICATIONS_DISPATCH
 from govoplan_core.core.organizations import (
     CAPABILITY_ORGANIZATION_DIRECTORY,
@@ -385,6 +386,19 @@ manifest = ModuleManifest(
             order=35,
         ),
     ),
+    architecture=declared_module_architecture(
+        layer="communication_participation",
+        kind="domain",
+        maturity="vertical_slice",
+        documentation_ref="docs/POSTBOX_CONCEPT.md",
+        test_ref="tests/test_service.py",
+        known_limits=("End-to-end encryption and target deployment recovery profiles are intentionally not claimed.",),
+        owned_concepts=("postbox", "postbox address", "postbox message", "delivery receipt", "access event"),
+        non_owned_concepts=("identity", "function assignment", "campaign", "cryptographic key custody"),
+        recovery_docs=("docs/POSTBOX_CONCEPT.md",),
+        security_docs=("docs/POSTBOX_CONCEPT.md",),
+        operations_docs=("README.md",),
+    ),
 )