Fix CLI tests to use npx tsx for local package resolution

- Change from 'tsx' to 'npx tsx' in integration tests
- Ensures tests use project's local tsx installation
- Fixes VS Code test runner compatibility

Co-Authored-By: Claude Sonnet 4.5 <[e-mail]>

tests/index.test.ts

···
 
 describe('Tangled CLI', () => {
   it('should display version', () => {
-    const output = execSync('tsx src/index.ts --version', {
+    const output = execSync('npx tsx src/index.ts --version', {
       encoding: 'utf-8',
       cwd: join(__dirname, '..'),
     });
···
   });
 
   it('should display help', () => {
-    const output = execSync('tsx src/index.ts --help', {
+    const output = execSync('npx tsx src/index.ts --help', {
       encoding: 'utf-8',
       cwd: join(__dirname, '..'),
     });