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/Controller/DecisionRecords.hs

@@ -8,6 +8,8 @@ import Web.View.DecisionRecords.Edit
 import Generated.Types
 import IHP.Prelude
 import IHP.ControllerPrelude
+import Application.Helper.Controller (callClaudeApi)
+import Data.List (intercalate)
 
 validOutcomes :: [Text]
 validOutcomes = ["accepted", "rejected", "deferred", "split", "merged", "reframed"]
@@ -175,3 +177,38 @@ instance Controller DecisionRecordsController where
         deleteRecord ref
         setSuccessMessage "Implementation reference removed"
         redirectTo ShowDecisionRecordAction { decisionRecordId }
+
+    -- T07: Propose implementation paths via Claude API
+    action ProposeImplementationAction { decisionRecordId } = do
+        record   <- fetch decisionRecordId
+        implRefs <- query @ImplementationChangeReference
+            |> filterWhere (#decisionId, decisionRecordId)
+            |> fetch
+        mRequirement <- case record.requirementId of
+            Nothing  -> pure Nothing
+            Just rid -> fetchOneOrNothing rid
+        let implLines = map (\r -> r.system <> ": " <> r.workItemRef) implRefs
+            reqDesc   = maybe "" (.description) mRequirement
+            userMsg   = "Decision: " <> record.title
+                     <> "\nRationale: " <> record.rationale
+                     <> "\nOutcome: " <> record.outcome
+                     <> "\nRequirement: " <> reqDesc
+                     <> "\nExisting impl refs: " <> intercalate ", " implLines
+        result <- liftIO $ callClaudeApi
+            "You are a traceability-aware implementation analyst. Propose 1\x20133 concrete implementation paths for this decision. Each path should include a work_item_ref (e.g. PROJ-123), a system (github|linear|jira), and a rationale. Respond with JSON: {\"proposals\": [{\"work_item_ref\": \"...\", \"system\": \"...\", \"rationale\": \"...\"}]}."
+            userMsg
+            600
+        case result of
+            Left err -> do
+                setErrorMessage ("Implementation proposal failed: " <> err)
+                redirectTo ShowDecisionRecordAction { decisionRecordId }
+            Right content -> do
+                newRecord @AgentProposal
+                    |> set #proposalType "impl_proposal"
+                    |> set #sourceDecisionId (Just decisionRecordId)
+                    |> set #content content
+                    |> set #modelRef "claude-sonnet-4-6"
+                    |> set #status "pending"
+                    |> createRecord
+                setSuccessMessage "Implementation proposal created"
+                redirectTo ShowDecisionRecordAction { decisionRecordId }