fix: correct state destructuring and document headless mode

- Fix extractCodeFromInput destructuring: returns { code, state } not
  { code, extractedState }, so state validation was being bypassed
- Add --no-browser hint to CLI banner for discoverability
- Document --no-browser mode in README.md and CLAUDE.md
- Add test:oauth script to package.json
- Add OAuth test to run-all.cjs test suite

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/run-all.cjs

@@ -14,7 +14,8 @@ const tests = [
     { name: 'Multi-turn Tools (Streaming)', file: 'test-multiturn-thinking-tools-streaming.cjs' },
     { name: 'Interleaved Thinking', file: 'test-interleaved-thinking.cjs' },
     { name: 'Image Support', file: 'test-images.cjs' },
-    { name: 'Prompt Caching', file: 'test-caching-streaming.cjs' }
+    { name: 'Prompt Caching', file: 'test-caching-streaming.cjs' },
+    { name: 'OAuth No-Browser Mode', file: 'test-oauth-no-browser.cjs' }
 ];
 
 async function runTest(test) {