docs: consolidate documentation and archive temporary reports

Step 1-2 (audit + plan): classified 35 project markdown files into
Core/Architecture/ADR/Temporary-audit/Sprint-report/Generated-review/
Duplicate/Obsolete/Historical. Agent-tooling files (.agents/skills/**,
.superpowers/**, docs/context/**, CLAUDE.md/GEMINI.md/AGENTS.md/
.github/copilot-instructions.md) explicitly out of scope — intentional
per-tool duplication, not documentation debt.

Step 3 (merge, no information lost):
- docs/PROJECT.md -> docs/PROJECT_INDEX.md, rewritten as the single
  entry point: system overview, living-doc index, archive pointer,
  current status, and a critical-finding callout up top.
- docs/backend/BACKEND-INTEGRATION.md -> docs/BACKEND_API.md,
  docs/backend/REMAINING-BACKEND-WORK.md ->
  docs/BACKEND_API_REMAINING_WORK.md (also folded in a legitimate
  uncommitted status update that had been sitting unstaged all
  session: categories marked DONE, order-creation endpoint noted done).
- RELEASE-NOTES.md merged into CHANGELOG.md (was a near-duplicate of
  the same release content in friendlier prose), then deleted.
- KNOWN-ISSUES.md: added item 13 (see below) and item 14 (missing
  canDeactivate on admin/products edit, from the archived PROJECT-STATE
  audit, re-verified still true); added a correction note to Fixed
  item 7.
- All cross-references to renamed/moved files fixed across every
  kept doc (grep+sed pass, then verified with a link-existence check
  across all 58 in-scope markdown files -> 0 broken links).

Step 4 (archive, nothing deleted without merging first): created
docs/archive/, moved 19 files there (3 root sprint reports, 1 platform
report, SPRINT-PLAN.md, and 14 one-off audit/review/report docs).
Added correction headers to the 3 archived docs whose conclusions were
affected by the finding below, rather than silently leaving them
misleading.

Step 5: docs/PROJECT_INDEX.md rewritten per the mission brief -
someone opening the repo should understand the whole system from it.

IMPORTANT FINDING (surfaced during this audit, not the mission's
primary goal but too significant to bury): pages/category/*,
pages/search/*, pages/item-detail/*, pages/info/**, pages/legal/**
(40+ files) are entirely unrouted dead code - app.routes.ts's
cmsContentRoutes is a literal empty array, and category/search/product
routes redirect to CatalogContainerComponent/
ProductDetailsContainerComponent, not these files. Confirmed against
app.routes.ts directly and cross-checked against FRONTEND.md's own
routing description. This means several fixes from earlier this cycle
(RC-Premium-01, RC STORE-01) and the dead-code cleanup sprint's
conclusion that these files were live were all wrong - documented as
KNOWN-ISSUES.md item 13, flagged at the top of PROJECT_INDEX.md, and
noted on the 3 archived docs whose conclusions it affects. No
application code was changed to fix this (out of scope per this
session's 'documentation only' constraint) - it needs a wire-it-up-or-
delete-it decision first.

Verification: tsc --noEmit clean, npm run build green, all markdown
links across 58 in-scope files resolve (checked programmatically).
No application/Angular/backend code modified.

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

docs/PROJECT-STRUCTURE.md

@@ -28,7 +28,7 @@ Standards referenced below are enforced, not suggestions: `docs/architecture/fou
 
 2. **Gateway interface** — `services/admin-dashboard-metrics.gateway.interface.ts`. An abstract contract (`AdminDashboardMetricsGateway`) for "however we get category/product counts" — deliberately decoupled from *how* (local computation vs. real API) so the facade never knows which implementation is active.
 
-3. **Gateway implementation** — `services/admin-dashboard-metrics.local.gateway.ts`. `AdminDashboardMetricsLocalGateway implements AdminDashboardMetricsGateway`, composing `BackofficeDataService.loadCategories()/loadProducts()` (already used elsewhere) into counts. A future `AdminDashboardMetricsApiGateway` would implement the same interface against a real endpoint (`docs/backend/BACKEND-INTEGRATION.md#615-backoffice--dashboard-metrics--recent-activity-planned`) — nothing above this layer changes when that happens.
+3. **Gateway implementation** — `services/admin-dashboard-metrics.local.gateway.ts`. `AdminDashboardMetricsLocalGateway implements AdminDashboardMetricsGateway`, composing `BackofficeDataService.loadCategories()/loadProducts()` (already used elsewhere) into counts. A future `AdminDashboardMetricsApiGateway` would implement the same interface against a real endpoint (`docs/BACKEND_API.md#615-backoffice--dashboard-metrics--recent-activity-planned`) — nothing above this layer changes when that happens.
 
 4. **DI token** — `services/admin-dashboard-metrics-gateway.token.ts`. `const ADMIN_DASHBOARD_METRICS_GATEWAY = new InjectionToken<AdminDashboardMetricsGateway>(...)`, bound to the local gateway by default in `app.config.ts`. This is the swap point: rebinding this token to a real API gateway is the *only* change needed to go from mock to real data.
 
@@ -56,5 +56,5 @@ Full narrative and known gaps: `docs/ADMIN.md`.
 8. Build the container/page component that injects the facade and wires routing.
 9. Add routes in `app.routes.ts`, with `adminAuthGuard` (or the relevant guard) if it's an admin surface.
 10. Add every new user-facing string to `i18n/translations.ts` (interface) then `en.ts`/`ru.ts`/`hy.ts` — never hardcode copy in a template.
-11. Document backend gaps (if any) in `docs/backend/BACKEND-INTEGRATION.md` (§6, endpoints by domain) using the same CURRENT/PLANNED/FUTURE tagging as the existing entries.
+11. Document backend gaps (if any) in `docs/BACKEND_API.md` (§6, endpoints by domain) using the same CURRENT/PLANNED/FUTURE tagging as the existing entries.
 12. Run `npm run arch:check` (import boundaries + circular dependencies) and `npx tsc -p tsconfig.app.json --noEmit` before committing.