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/schema.test.js

@@ -181,6 +181,13 @@ describe("graphUpdateSchema", () => {
         question: "What does this new node mean?",
         reason: "A follow-up unknown remains.",
       },
+      answerMeaning: {
+        userSupportedMeaning:
+          "The user directly established a concrete answer.",
+        possibleInference: null,
+        supportCategory: "other",
+        resolutionGuidance: "may_resolve",
+      },
     });
     expect(result.success).toBe(true);
   });
@@ -192,6 +199,26 @@ describe("graphUpdateSchema", () => {
     expect(result.success).toBe(true);
   });
 
+  it("allows null answerMeaning", () => {
+    const result = graphUpdateSchema.safeParse({
+      answerMeaning: null,
+    });
+    expect(result.success).toBe(true);
+  });
+
+  it("validates structured answerMeaning when present", () => {
+    const result = graphUpdateSchema.safeParse({
+      answerMeaning: {
+        userSupportedMeaning: "Risk matters more to me.",
+        possibleInference:
+          "This may imply caution, but does not establish a hard constraint.",
+        supportCategory: "relative_priority_only",
+        resolutionGuidance: "must_remain_unresolved",
+      },
+    });
+    expect(result.success).toBe(true);
+  });
+
   it("rejects update with invalid node kind in addedNodes", () => {
     const invalid = graphUpdateSchema.safeParse({
       addedNodes: [