docs: add documentation index scaffold

- Create docs/README.md with YAML frontmatter and index placeholder
- Add Documentation (SOURCE OF TRUTH) section to root CLAUDE.md

docs/README.md

@@ -0,0 +1,13 @@
+---
+title: "Documentation"
+role: part
+parent: null
+order: 1
+description: "Project documentation index"
+---
+
+# Documentation
+
+Project documentation lives here.
+
+> This directory is maintained automatically. When features are added or changed, corresponding documentation is updated.