_terminate_owned_chrome() walked Path("/proc") unconditionally, so on macOS
and Windows iterdir() raised FileNotFoundError out of private-browser session
shutdown. src/tools/cookbook.py already guards the same kind of scan with
os.path.isdir("/proc").
The sweep only reclaims Chrome trees that agent-browser reparented, so it is
an optimisation rather than a correctness requirement: degrade to a no-op
rather than failing the whole shutdown path.
_PROC_ROOT is a module attribute so both branches are testable on either kind
of host. The procfs-present path had no coverage at all before this.
Léo