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>

src/app/shared/ui/choice-list/choice-list.stories.ts

@@ -21,7 +21,7 @@ const meta: Meta<ChoiceListComponent> = {
   }),
   parameters: {
     // Structural: app-choice-link's host sits between the <ul> and its <li> —
-    // fixed by the WP-11 markup rework. See docs/backlog/WP-11-markup-fidelity.md.
+    // fixed by the WP-11 markup rework. See docs/project/backlog/WP-11-markup-fidelity.md.
     a11y: { disable: true },
   },
 };