feat(admin): add posts CRUD sandbox and shared CRUD foundation

docs/product-engineering/crud-baseline.md

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+# CRUD Baseline
+
+## Scope
+
+MVP0 now includes a shared CRUD foundation package: `@cms/crud`.
+
+Current baseline:
+
+- Shared service factory: `createCrudService`
+- Shared validation error type: `CrudValidationError`
+- Shared not-found error type: `CrudNotFoundError`
+- Shared mutation audit hook contract: `CrudAuditHook`
+- Shared mutation context contract (`actor`, `metadata`)
+
+## First Integration
+
+`@cms/db` `posts` now uses the shared CRUD foundation:
+
+- `listPosts`
+- `getPostById`
+- `createPost`
+- `updatePost`
+- `deletePost`
+- `registerPostCrudAuditHook`
+
+Validation for create/update is enforced by `@cms/content` schemas.
+
+The admin dashboard currently includes a temporary posts CRUD sandbox to validate this flow through a real app UI.
+
+## Notes
+
+- This is the base layer for future entities (pages, navigation, media, users, commissions).
+- Audit hook persistence/transport is intentionally left for later implementation work.