refactor: implement script tag approach for library inclusion

- Add script tags to demo-site HTML files for manual development
- Disable CLI inline script injection to prevent duplicate scripts
- Add library serving endpoints to servedev command
- Update build process to auto-copy library to demo-site
- Add CDN URL helpers for future production deployment
- Update .gitignore for generated demo-site files

Fixes .insertr-gate authentication for manual npm run serve workflow
while maintaining clean separation between CLI and manual setups.

insertr-cli/scripts/rebuild-library.sh

@@ -7,7 +7,7 @@ set -e
 
 echo "🔄 Library changed, rebuilding..."
 
-# Build the library
+# Build the library (this will also copy to demo-site)
 cd ../lib
 npm run build --silent