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>

tests/test_suggestions.py

@@ -83,7 +83,11 @@ class TestSuggestionsRetired:
         r = await client.post(f"/suggestions/{sid}/bump-relevance", json={})
         assert r.status_code == 410
 
-    async def test_get_and_list_still_work_for_historical_rows(self, client, test_engine):
+    async def test_reads_are_retired_too(self, client, test_engine):
+        """Slice E1 (STATE-WP-0079-T05, 2026-08-20): reads were kept live only
+        to keep the historical backlog reachable. That history is archived at
+        the-custodian/docs/archived-suggestion-backlog.md, so the read surface
+        is retired as well."""
         await _create_domain(client, "custodian", "Custodian")
         sid = await _seed_suggestion(
             test_engine, domain_slug="custodian",
@@ -91,16 +95,17 @@ class TestSuggestionsRetired:
             origin="WARDEN-WP-0012", origin_ref="example",
         )
 
-        r = await client.get(f"/suggestions/{sid}")
-        assert r.status_code == 200
-        assert r.json()["title"] == "Historical gated-need signal"
+        for path in (f"/suggestions/{sid}", "/suggestions/", f"/suggestions/{sid}/notes"):
+            r = await client.get(path)
+            assert r.status_code == 410, path
 
-        r2 = await client.get("/suggestions/")
-        assert r2.status_code == 200
-        assert any(s["id"] == sid for s in r2.json())
-
-    async def test_unknown_id_still_404s_not_410(self, client):
-        """Read paths keep their normal semantics; only mutations are
-        blanket-retired."""
+    async def test_unknown_id_also_410s(self, client):
+        """The whole prefix is retired, so there is nothing left to 404 on."""
         r = await client.get(f"/suggestions/{uuid.uuid4()}")
-        assert r.status_code == 404
+        assert r.status_code == 410
+
+    async def test_retirement_detail_points_at_intake_and_archive(self, client):
+        r = await client.get("/suggestions/")
+        detail = r.json()["detail"]
+        assert "POST /intakes/" in detail
+        assert "archived-suggestion-backlog.md" in detail