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/prompt-builder.test.js

@@ -73,6 +73,7 @@ describe("buildGraphUpdatePrompt", () => {
     expect(prompt).toContain("resolvedUnknownNodeIds");
     expect(prompt).toContain("affectedNodeIds");
     expect(prompt).toContain("selectedQuestion");
+    expect(prompt).toContain("answerMeaning");
   });
 
   it("lists enum values", () => {
@@ -111,4 +112,17 @@ describe("buildGraphUpdatePrompt", () => {
       "the engine will deterministically choose final priority after validation",
     );
   });
+
+  it("instructs the model to preserve user-supported meaning separately from inference", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    expect(prompt).toContain(
+      "answerMeaning.userSupportedMeaning must state only what the user's answer directly supports",
+    );
+    expect(prompt).toContain(
+      "Put any stronger interpretation in answerMeaning.possibleInference",
+    );
+    expect(prompt).toContain(
+      "If the answer is only a relative priority statement, use supportCategory=relative_priority_only and resolutionGuidance=must_remain_unresolved",
+    );
+  });
 });