refactor: consolidate all Node.js development into lib package

- Move scripts/ to lib/scripts/ and convert to ESM modules
- Consolidate dependencies: add live-server to lib/package.json
- Remove root package.json and node_modules split
- Preserve CLI integration via existing rebuild-library.sh
- Add development quickstart guide for new unified workflow
- Clean up outdated file references and duplicate assets

lib/src/core/insertr.js

@@ -12,7 +12,7 @@ export class InsertrCore {
     
     // Find all enhanced elements on the page
     findEnhancedElements() {
-        return document.querySelectorAll('[data-insertr-enhanced="true"]');
+        return document.querySelectorAll('.insertr');
     }
     
     // Get element metadata