Add enterprise docs structure and include pending core maintenance updates.

docs/README.md

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+# CBDD Documentation
+
+This folder is the canonical source for internal operational and engineering documentation.
+
+## Core Documents
+
+- Architecture: [`architecture.md`](architecture.md)
+- Deployment: [`deployment.md`](deployment.md)
+- Operations runbook: [`runbook.md`](runbook.md)
+- Security controls: [`security.md`](security.md)
+- Access model: [`access.md`](access.md)
+- Troubleshooting guide: [`troubleshooting.md`](troubleshooting.md)
+
+## Major Features
+
+- Feature inventory: [`features/README.md`](features/README.md)
+
+## Architecture Decisions
+
+- Initial ADR: [`adr/0001-storage-engine-and-source-generation.md`](adr/0001-storage-engine-and-source-generation.md)
+
+## Reference Specifications
+
+- Engine RFC: [`../RFC.md`](../RFC.md)
+- C-BSON format spec: [`../C-BSON.md`](../C-BSON.md)