fix: properly mock os module in custom-system-prompt test

- Add default export to os module mock to fix vitest error
- This resolves the test failure on both Ubuntu and Windows CI
- The issue was not related to toPosix but to improper os module mocking

Author: roomote

src/core/prompts/__tests__/custom-system-prompt.spec.ts

@@ -49,6 +49,10 @@ vi.mock("../../../services/code-index/manager", () => ({
 }))
 
 vi.mock("os", () => ({
+	default: {
+		type: vi.fn().mockReturnValue("Windows_NT"),
+		homedir: vi.fn().mockReturnValue("/home/test"),
+	},
 	type: vi.fn().mockReturnValue("Windows_NT"),
 	homedir: vi.fn().mockReturnValue("/home/test"),
 }))