reasoning: add proposal-level answer meaning guard

- Add answerMeaning schema with supportCategory and resolutionGuidance enums
- Add pre-mutation guard that validates proposal alignment with answerMeaning
- Update prompt builder to instruct the model on answerMeaning contract
- Add tests for schema, guard logic, parsing defaults, and regression cases A-D

tests/graph/update-proposal.test.js

@@ -19,6 +19,13 @@ function makeValidProposal(overrides = {}) {
     resolvedUnknownNodeIds: ["n-unknown"],
     affectedNodeIds: [],
     selectedQuestion: null,
+    answerMeaning: {
+      userSupportedMeaning:
+        "The user directly provided the updated complaint rate.",
+      possibleInference: null,
+      supportCategory: "other",
+      resolutionGuidance: "may_resolve",
+    },
     ...overrides,
   };
 }
@@ -131,6 +138,20 @@ describe("parseGraphUpdateProposal", () => {
     expect(result.proposal.selectedQuestion).toBeNull();
   });
 
+  it("defaults missing answerMeaning to null", () => {
+    const result = parseGraphUpdateProposal({
+      addedNodes: [],
+      updatedNodes: [],
+      addedEdges: [],
+      removedEdgeIds: [],
+      resolvedUnknownNodeIds: [],
+      affectedNodeIds: [],
+      selectedQuestion: null,
+    });
+    expect(result.success).toBe(true);
+    expect(result.proposal.answerMeaning).toBeNull();
+  });
+
   it("parses a valid selectedQuestion", () => {
     const result = parseGraphUpdateProposal(
       makeValidProposal({