Add automated ISAC system notifications

docs/architecture.md

@@ -342,3 +342,29 @@ window. Daily rows are assigned to the next 10:00 AM wall-clock boundary in
 dispatched rows retain `delivered_at` evidence and are never selected again.
 The existing ISAC notification delivery preferences still determine optional
 browser and email fan-out after the consolidated inbox message is created.
+
+## Automated ISAC system messages
+
+Automated message definitions live in `automated_system_messages` and are
+managed through ISAC → System Configuration → Automated Messages by agents with
+`system.notifications.manage`. Edits are recorded under the
+`system_configuration` audit category. Definitions identify a lifecycle event,
+enabled state, delay in minutes, message content, primary destination, and
+structured action callouts.
+
+`account.created` is emitted by password, magic-link, and Discord registration.
+The delivery table has a unique message/user/event/event-ID constraint, so the
+same account creation cannot deliver the Welcome Agent message more than once.
+The default welcome introduces News, Database, Builds, Guides, Builder, ISAC
+Contributor Mode, and the SHD Discord community.
+
+Immediate definitions are delivered in the registration request. Definitions
+with a delay are persisted in `automated_system_message_deliveries`. Production
+must run the delayed dispatcher once per minute:
+
+```text
+* * * * * cd /path/to/shd.build && npm run notifications:system:production
+```
+
+Only one dispatcher instance should run for the embedded database. Delivered
+rows retain both their delivery timestamp and generated notification ID.