feat(WP-67): merge behandelportal into this repo as a monorepo

Restructures into apps/ssp + apps/behandelportal (two Angular projects)
plus libs/shared + libs/beheer (cross-app libraries), replacing WP-61's
separate sibling repo. That split had already produced real drift: a
hand-vendored copy of the backend's OpenAPI doc, a shared/ui+layout tree
forked and silently diverging (7 files), and beheer + the styles.scss
token bridge duplicated byte-for-byte across both repos.

- git mv the SSP's src/app/* into apps/ssp/; fold shared/, beheer/,
  environments/, the Storybook docs/*.mdx, and styles.scss into
  libs/shared + libs/beheer (all confirmed identical between the two
  repos before merging). auth stays deliberately duplicated per
  ADR-0002 (actor-specific, expected to diverge) - amended there.
- One generated API client (libs/shared), no more vendored swagger.json.
- .dependency-cruiser split into a base factory + one config per app,
  and Storybook into .storybook-ssp/.storybook-behandelportal - both
  forced by the @auth/* alias resolving to different directories per app.
- SiteHeaderComponent/ShellComponent gained HEADER_NAV_ITEMS/
  HEADER_ADMIN_LINKS/DEBUG_PANEL injection tokens so each app supplies
  its own nav/admin-links/dev-panel instead of one being hardcoded.
- CLAUDE.md, ARCHITECTURE.md, dependencies.md, and ADR-0002 updated;
  WP-67 backlog entry documents the full decision trail.

npm run ci green (lint, dep:check x2, 360 tests across ssp/
behandelportal/shared/beheer, both localized builds, backend tests,
snippet + api-client drift); both dev servers, both Storybook
instances, and docker compose verified working.

The old sibling repo (/home/eho/repos/behandelportal) is left
untouched, not deleted.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

libs/shared/docs/overview.mdx

@@ -0,0 +1,39 @@
+import { Meta } from '@storybook/addon-docs/blocks';
+
+<Meta title="Foundations/Overview" />
+
+# Foundations
+
+The **why** behind this codebase, as a short curriculum. Each page is a condensed,
+cross-linked take on one idea; the long-form source lives in `docs/reference/`
+(see the repo's `docs/README.md`). Read them in roughly this order.
+
+> **New here?** Follow the [Learning Path](?path=/docs/foundations-learning-path--docs)
+> for a paced 3-day route with exercises and self-checks. This page is the map; the
+> Learning Path is the route through it.
+
+## Architecture & domain
+
+- [Domain-driven design](?path=/docs/foundations-domain-driven-design--docs) — bounded
+  contexts + layers, dependencies pointing inward (the folder structure _is_ the architecture).
+- [Atomic design](?path=/docs/foundations-atomic-design--docs) — Atoms → Molecules →
+  Organisms → Templates; a new page is composition, not new building blocks.
+
+## Functional core
+
+- [FP in the UI](?path=/docs/foundations-fp-in-the-ui--docs) — the three functional tools behind the view.
+- [State machines (TEA)](?path=/docs/foundations-state-machines-tea--docs) — every form/wizard as Model → Msg → pure `reduce`.
+- [RemoteData & Async](?path=/docs/foundations-remotedata-async--docs) — the four async states as one value.
+- [Parse, don't validate](?path=/docs/foundations-parse-dont-validate--docs) — narrow untrusted `unknown` at the boundary into domain types.
+
+## Design system
+
+- [Design tokens](?path=/docs/foundations-design-tokens--docs) — semantic `--rhc-*` tokens; no hand-written colours.
+- [CIBG gap register](?path=/docs/foundations-cibg-gap-register--docs) — where we diverge from the CIBG Huisstijl (ADR-0003).
+
+## Quality & process
+
+- [Accessibility](?path=/docs/foundations-accessibility--docs) — four layered a11y tools, each catching a different bug class.
+- [Testing strategy](?path=/docs/foundations-testing-strategy--docs) — what to test, by layer grain.
+- [BDD](?path=/docs/foundations-bdd--docs) — how each test is phrased and scoped: one behaviour, in the domain's language.
+- [Internationalization](?path=/docs/foundations-internationalization--docs) — `$localize` for every user-visible string; the locale seam.