docs(adr): add glossary pages and ADR baseline structure

docs/product-engineering/domain-glossary.md

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+# Domain Glossary
+
+## Core Terms
+
+### Owner
+
+Highest-privilege admin role. Exactly one canonical owner must exist at all times.
+
+### Support User
+
+Hidden technical support account used for break-glass access and operational recovery.
+
+### Admin Registration Policy
+
+Runtime policy controlling whether `/register` can create additional admin users after owner bootstrap.
+
+### Protected Account
+
+Account that cannot be deleted/demoted through self-service flows (support + canonical owner).
+
+### CRUD Service
+
+Shared `@cms/crud` service abstraction combining schema validation, repository orchestration, and audit hooks.
+
+### Permission Scope
+
+RBAC access scope granularity: `own`, `team`, `global`.
+
+### Roadmap Source Of Truth
+
+`TODO.md` in repository root. Rendered in admin via `/todo`.
+
+### Header Banner
+
+Public-site announcement strip configured through `system_setting` key `public.header_banner`.