fix: prevent agent subprocess blocking on Windows

- Add stdin=subprocess.DEVNULL to prevent blocking on stdin reads
- Add CREATE_NO_WINDOW flag on Windows to prevent console pop-ups
- Remove trailing pause from start_ui.bat

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

start_ui.bat

@@ -39,5 +39,3 @@ pip install -r requirements.txt --quiet
 
 REM Run the Python launcher
 python "%~dp0start_ui.py" %*
-
-pause