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>

backend/README.md

@@ -2,7 +2,7 @@
 
 The backend that hosts the **business rules** for the BIG-register portal. The
 frontend renders the decisions this service computes; it does not recompute them
-(BFF-lite + decision DTOs — see `../docs/architecture/0001-bff-lite-decision-dtos.md`).
+(BFF-lite + decision DTOs — see `../docs/reference/architecture/0001-bff-lite-decision-dtos.md`).
 
 No real BRP/DUO: the reference data they'd return (registration, person, diplomas,
 notes — `Data/SeedData.cs`) is in-memory and seeded, but the endpoints, DTOs,
@@ -15,7 +15,7 @@ status codes and error envelope are production-shaped.
 covers it, see `docker-compose.yml`) does **not** lose data. Delete the file to
 reset demo data back to empty, the same state a fresh clone starts from. This is
 a deliberate, right-sized choice for a POC (SQLite, no external DB service) — see
-`docs/backlog/WP-22-durable-persistence.md`.
+`docs/project/backlog/WP-22-durable-persistence.md`.
 
 ## Run