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>

api/services/write_idempotency.py

@@ -45,11 +45,6 @@ WRITE_ROUTE_RULES: tuple[WriteRouteRule, ...] = (
     WriteRouteRule("POST", r"/decisions/[^/]+/resolve", "replace", "resolve decision"),
     WriteRouteRule("PATCH", r"/workplans/[^/]+", "replace", "update workplan"),
     WriteRouteRule("PATCH", r"/workstreams/[^/]+", "replace", "update legacy workstream alias"),
-    WriteRouteRule("POST", r"/suggestions", "append", "create suggestion"),
-    WriteRouteRule("POST", r"/suggestions/[^/]+/vet", "replace", "vet suggestion"),
-    WriteRouteRule("POST", r"/suggestions/[^/]+/decline", "replace", "decline suggestion"),
-    WriteRouteRule("POST", r"/suggestions/[^/]+/promote", "replace", "promote suggestion to task"),
-    WriteRouteRule("POST", r"/suggestions/[^/]+/bump-relevance", "append", "bump suggestion relevance"),
 )