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-08-store-idiom.md

@@ -5,7 +5,7 @@ Phase: 1 — FP/DDD core
 
 ## Why
 
-Docs (`docs/fp-tea-atomic-design.md`, ARCHITECTURE §2c) teach `createStore` as THE
+Docs (`docs/reference/fp-tea-atomic-design.md`, ARCHITECTURE §2c) teach `createStore` as THE
 wiring, but the wizard pages hand-wire `signal(model)` + local `dispatch()` — juniors see
 two idioms and copy the wrong one. Machine naming also drifts:
 `change-request.machine.ts` exports bare `State`/`Msg`; `upload.machine.ts` exports
@@ -14,7 +14,7 @@ two idioms and copy the wrong one. Machine naming also drifts:
 ## Read first
 
 - `src/app/shared/application/store.ts` (`createStore`) + its spec
-- `docs/fp-tea-atomic-design.md` Part on TEA-in-Angular
+- `docs/reference/fp-tea-atomic-design.md` Part on TEA-in-Angular
 - All `*.machine.ts` files (6) and their UI wiring sites (wizard components/pages)
 
 ## Decisions (pre-made, don't relitigate)