persistent_local_browser broken on Windows: signal.SIGTERM and start_new_session are POSIX-only

Description

persistent_local_browser.py has two Windows incompatibilities that prevent the tool from working on Windows:

1. signal.SIGTERM not available on Windows (line 187): os.kill(pid, signal.SIGTERM) raises AttributeError because signal.SIGTERM is a Unix-only constant. The release command crashes.

2. start_new_session is POSIX-only (line 141-142): The process detach logic only applies on POSIX. On Windows, the Chromium subprocess is not detached from the parent, so it gets killed when the parent exits - making the "persistent" browser feature non-functional.

Impact

• The entire persistent_local_browser tool is broken on Windows
• _cmd_release crashes with AttributeError when trying to terminate a browser
• Persistent browser sessions cannot outlive the creating script on Windows
• Error message is a cryptic Python traceback, not a helpful diagnostic

Suggested Fix

Add Windows code paths:

• For process detach: use creationflags=subprocess.CREATE_NEW_PROCESS_GROUP on Windows
• For termination: use taskkill /F /T /PID on Windows as an alternative to os.kill(pid, signal.SIGTERM)

Start in persistent_local_browser.py at the process creation logic around lines 141-142 and the release termination logic around line 187. Check the Windows subprocess behavior and the existing release command, then verify that a Chromium session survives the parent exit and that release terminates it without an AttributeError.