refactor(FN-2161): standardize on createFnAgent naming

- Rename engine export and call sites to use createFnAgent consistently across runtime flows
- Update core lazy engine loader and dashboard agent-generation/planning/chat paths to reference createFnAgent
- Refresh affected unit and integration tests, including renaming pi-create-kb-agent.test.ts to pi-create-fn-agent.test.ts
- Update AGENTS.md documentation references to match the new createFnAgent name

packages/dashboard/src/milestone-slice-interview.test.ts

@@ -7,7 +7,7 @@ const { mockCreateKbAgent } = vi.hoisted(() => ({
 }));
 
 vi.mock("@fusion/engine", () => ({
-  createKbAgent: mockCreateKbAgent,
+  createFnAgent: mockCreateKbAgent,
 }));
 
 import {
@@ -217,7 +217,7 @@ describe("milestone-slice-interview module", () => {
   beforeEach(async () => {
     vi.clearAllMocks();
     __resetMilestoneSliceInterviewState();
-    // Reset cached createKbAgent to force re-import with mock
+    // Reset cached createFnAgent to force re-import with mock
     const mod = await import("./milestone-slice-interview.js") as any;
     mod.__resetEngine?.();
     mockCreateKbAgent.mockImplementation(async () => createMockAgent([createQuestionJson()]));