feat(P5): IHF Phase 5 complete — agent-assisted distillation

Adds bounded AI support to the IHF governance loop. All AI outputs are
attributed (model_ref), reviewable (AgentReviewRecord), and reversible.
No autonomous decisions; no silent requirement promotion.

- T01: Schema — agent_proposals, agent_review_records,
  confidence_annotations (migration 1743379200)
- T02: AgentProposalsController (index/show/accept/reject, idempotent
  review guard), global nav "Agent" link
- T03: SummarizeClusterAction — Claude API cluster summary on widget show
- T04: DraftRequirementAction — AI requirement draft; acceptance creates
  RequirementCandidate (human-gated)
- T05: DetectDuplicatesAction — duplicate_flag proposal on candidate show
- T06: DetectPolicySensitivityAction — policy_flag with
  ConfidenceAnnotations per concern scope
- T07: ProposeImplementationAction — impl_proposal from decision show
- T08: AgentAuditDashboardAction — autoRefresh; KPI row, unreviewed queue,
  recent proposals, attribution log matrix
- T09: integration tests, SCOPE.md updated, phase5-summary.md, flake.nix
  adds http-conduit/aeson/string-conversions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Web/View/DecisionRecords/Show.hs

@@ -114,7 +114,14 @@ instance View ShowView where
 
             <!-- Implementation references -->
             <div class="bg-white rounded-lg border border-gray-200 px-6 py-4">
-                <h2 class="text-sm font-semibold text-gray-700 mb-3">Implementation References</h2>
+                <div class="flex items-center justify-between mb-3">
+                    <h2 class="text-sm font-semibold text-gray-700">Implementation References</h2>
+                    <form method="POST" action={ProposeImplementationAction { decisionRecordId = record.id }} class="inline">
+                        <button type="submit" class="text-xs border border-green-300 text-green-700 px-2 py-1 rounded hover:bg-green-50">
+                            Propose Implementation
+                        </button>
+                    </form>
+                </div>
                 {forEach implRefs renderImplRef}
                 <form method="POST" action={AddImplementationRefAction { decisionRecordId = record.id }}
                       class="mt-3 flex items-end gap-2">