fix: explicitly forbid extra frontmatter fields in output format instruction

buildOutputFormatInstruction now includes explicit language telling agents to
output ONLY schema-defined fields and to focus on their role's deliverable.

Fixes #394

packages/workflow-agent-kit/__tests__/build-output-format-instruction.test.ts

@@ -141,6 +141,28 @@ describe("buildOutputFormatInstruction", () => {
     expect(result).toContain("shared: <string>  # required");
   });
 
+  test("explicitly forbids extra frontmatter fields", () => {
+    const result = buildOutputFormatInstruction(PLANNER_SCHEMA);
+    expect(result).toMatch(/\b(only|exclusively)\b.*fields/i);
+    expect(result).toMatch(/do not add (extra|additional|other) fields/i);
+  });
+
+  test("forbids extra fields even for empty schema", () => {
+    const result = buildOutputFormatInstruction({});
+    expect(result).toMatch(/do not add (extra|additional|other) fields/i);
+  });
+
+  test("forbids extra fields for anyOf/oneOf schemas", () => {
+    const schema = {
+      anyOf: [
+        { type: "object", properties: { alpha: { type: "string" } } },
+        { type: "object", properties: { beta: { type: "number" } } },
+      ],
+    };
+    const result = buildOutputFormatInstruction(schema);
+    expect(result).toMatch(/do not add (extra|additional|other) fields/i);
+  });
+
   test("includes focus reminder about role scope", () => {
     const result = buildOutputFormatInstruction({});
     expect(result).toContain("Focus exclusively on YOUR role");

packages/workflow-agent-kit/src/build-output-format-instruction.ts

@@ -191,5 +191,7 @@ Your meta output must satisfy these fields:
 
 ${fieldList}
 
+Output ONLY the fields listed above. Do not add extra fields that are not specified in the schema.
+
 Focus exclusively on YOUR role's deliverable. Do not perform actions outside your role's scope.`;
 }