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>

docs/project/backlog/WP-28-brief-v2-demo-polish.md

@@ -22,7 +22,7 @@ that keep CLAUDE.md and the backlog truthful.
   already covers all `/api/` calls, the new endpoints included), and
   `POST /brief/reset`. The demo script documents the mapping; no new interceptor
   cases, no scenario code.
-- Demo script lives at `docs/prd/0003-brief-v2-demo-script.md` and follows the §6
+- Demo script lives at `docs/project/prd/0003-brief-v2-demo-script.md` and follows the §6
   choreography (compose → preview → switch sub-org seed → "two axes, one render").
 - One e2e spec, not a suite: drafter composes on canvas → submit → approve → send
   pins the org-template version; admin publishes → drafter canvas reflects it.
@@ -31,10 +31,10 @@ that keep CLAUDE.md and the backlog truthful.
 
 ## Files
 
-- `docs/prd/0003-brief-v2-demo-script.md` (new)
+- `docs/project/prd/0003-brief-v2-demo-script.md` (new)
 - `e2e/brief-v2.spec.ts` (new)
 - story gap-fill where WP-24..27 left holes
-- `docs/backlog/README.md` (statuses), `CLAUDE.md` (roles/routes touch-up)
+- `docs/project/backlog/README.md` (statuses), `CLAUDE.md` (roles/routes touch-up)
 
 ## Steps