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>

dashboard/src/wsjf-triage.md

@@ -219,7 +219,7 @@ injectTocTop("live-indicator", _liveEl);
 const _h1 = document.querySelector("#observablehq-main h1");
 if (_h1) { _h1.style.position = "relative"; withDocHelp(_h1, "/docs/wsjf-triage"); }
 
-display(html`<p class="triage-subtitle">Daily State Hub triage from activity-core. Recommendations are advisory; the operator and workplan owners decide what to act on. Ranked <a href="/suggestions">suggestion backlog</a> feeds the digest.</p>`);
+display(html`<p class="triage-subtitle">Daily State Hub triage from activity-core. Recommendations are advisory; the operator and workplan owners decide what to act on. The ranked suggestion backlog that once fed this digest is retired (slice E1); the digest now stands on workplan and task state alone.</p>`);
 display(html`<div class="triage-latest">
   <span>Last updated</span>
   <strong>${latestReport ? fmtDateTime(latestReport.created_at) : "No daily_triage events yet"}</strong>