docs: reorganize into project/ + reference/, expand Storybook Foundations

Move working docs (backlog, prd, roadmap) under docs/project/ and durable
docs (architecture ADRs, guides, audits) under docs/reference/; add a
docs/README.md index. Update every path reference in code comments, CLAUDE.md,
READMEs, and the new-ssp skill. Expand the Storybook Foundations curriculum
(Overview, BDD, i18n; rename Layers→Domain-Driven Design) and reorder the sidebar.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.storybook/preview.ts

@@ -31,6 +31,21 @@ const preview: Preview = {
       storySort: {
         order: [
           'Foundations',
+          [
+            'Overview',
+            'Domain-Driven Design',
+            'Atomic Design',
+            'FP in the UI',
+            'State Machines (TEA)',
+            'RemoteData & Async',
+            "Parse, don't validate",
+            'Design Tokens',
+            'CIBG Gap Register',
+            'Accessibility',
+            'Testing strategy',
+            'BDD',
+            'Internationalization',
+          ],
           'Design System',
           ['Atoms', 'Molecules', 'Organisms', 'Templates', 'Devtools'],
           'Domein',