feat: add docs/ folder (architecture, guides, ADRs)

docs/decisions/adr-003-cms-separation.md

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+# ADR-003: CMS Core + CMS Client Separation
+
+## Status: Accepted
+
+## Context
+
+Payload CMS needs to integrate with clean architecture without creating circular dependencies.
+
+## Decision
+
+Three packages: `@repo/cms-core` (config + collections), `@repo/cms-client` (standalone typed client), `apps/cms` (thin shell).
+
+## Rationale
+
+- `cms-core` is testable independently — collections are just config objects
+- `cms-client` is standalone (no internal imports) — prevents circular deps
+- `apps/cms` is almost empty — just boots Next.js with cms-core's config
+- Payload instance injected into cms-client, not imported — app startup code wires them
+
+## Circular Dependency Prevention
+
+```
+apps/cms → @repo/cms-core → @repo/core/application (hooks)
+@repo/core/infrastructure → @repo/cms-client (standalone)
+```
+
+No cycles because cms-client never imports from cms-core or core.