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>

CLAUDE.md

@@ -1,8 +1,8 @@
 # CLAUDE.md
 
-Agent guide for this repo. The _why_ lives in `docs/ARCHITECTURE.md`,
-`docs/architecture/0001-bff-lite-decision-dtos.md`, and the learning guide
-`docs/fp-tea-atomic-design.md` (FP + The Elm Architecture + atomic design); this
+Agent guide for this repo. The _why_ lives in `docs/reference/architecture/ARCHITECTURE.md`,
+`docs/reference/architecture/0001-bff-lite-decision-dtos.md`, and the learning guide
+`docs/reference/fp-tea-atomic-design.md` (FP + The Elm Architecture + atomic design); this
 file is the _rules_. When a decision below and those docs disagree, the docs win —
 update this file.
 
@@ -12,7 +12,7 @@ signals. Auth is faked; **data and business rules are served by a minimal ASP.NE
 Core backend** (`backend/`, see its README) and consumed through an NSwag-generated
 typed client. The FE renders the backend's decisions. Reference data mimicking
 BRP/DUO (`Data/SeedData.cs`) is in-memory; applications, documents and the brief
-persist to a SQLite file via EF Core (WP-22) — `docs/backlog/WP-22-durable-persistence.md`.
+persist to a SQLite file via EF Core (WP-22) — `docs/project/backlog/WP-22-durable-persistence.md`.
 
 ## Commands