refactor: split cli and library entry

tests/library-exports.test.ts

@@ -0,0 +1,9 @@
+import { describe, expect, it } from 'vitest';
+import { resolveCredentials, TwitterClient } from '../src/index.js';
+
+describe('library exports', () => {
+  it('exposes primary library surface', () => {
+    expect(typeof TwitterClient).toBe('function');
+    expect(typeof resolveCredentials).toBe('function');
+  });
+});