feat(STATE-WP-0079): retire the suggestion-backlog surface (slice E1)

History is archived fleet-side, so the read surface has no remaining job.

- api/routers/suggestions.py: whole prefix 410s with a pointer to intake and
  to the archive; the router drops from 176 lines to a stub
- mcp_server: the 6 suggestion tools removed outright rather than stubbed --
  a retired tool that still appears in the tool list costs every agent
  session context on every call, which is the opposite of retiring it
- write_idempotency: 5 /suggestions rules dropped
- dashboard: suggestions.md deleted, nav entry removed, reference.md and
  wsjf-triage.md updated; docs/suggestions.md rewritten as archive pointer
- tests: two tests pinned the old read-live behaviour and now pin the
  retirement contract instead

Tables stay: they are retire/archive in SHR-INV-0001 and are captured by the
final dump at T06.

Untouched, and worth knowing during cutover: ui-feedback.md / todo.md
'suggestions' are Shift+click dashboard feedback backed by technical_debt, a
different entity that shares the word. E3 (dashboard-meta) is that page; its
owner is state-hub-until-cutover so it retires at the T06 window, not now.

Full suite 612 passed.

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

docs/evidence/legacy-meter-weekly-review-20260820.json

@@ -1,12 +1,12 @@
 {
-  "captured_at": "2026-08-20T05:27:26.224218+00:00",
+  "captured_at": "2026-08-20T06:00:07.693350+00:00",
   "api_base": "http://127.0.0.1:8000",
   "workplan": "STATE-WP-0070",
   "retired_interfaces": [],
   "weekly_review": {
-    "generated_at": "2026-08-20T05:27:26.217390Z",
-    "window_start": "2026-08-13T05:27:26.168151Z",
-    "window_end": "2026-08-20T05:27:26.168151Z",
+    "generated_at": "2026-08-20T06:00:07.687814Z",
+    "window_start": "2026-08-13T06:00:07.633734Z",
+    "window_end": "2026-08-20T06:00:07.633734Z",
     "cadence": "weekly",
     "activity_core_handoff": {
       "activity_id": "statehub-legacy-interface-review",