refactor(vr): route headset audio with a WirePlumber hook

A default-node hook selects wivrn.sink/wivrn.source whenever they exist,
so the polling wivrn-audio.nu unit, its saved state and `just vr-audio`
go away. Streams follow via linking.follow-default-target.

Assisted-by: Claude Code (claude-opus-5-5)

Justfile

@@ -366,26 +366,16 @@ game:
 del-failed:
   kubectl delete pod --all-namespaces --field-selector="status.phase==Failed"
 
-# Start PC VR on idols-ai and route desktop audio to the headset.
+# Start PC VR on idols-ai; WirePlumber routes audio to the headset once it connects.
 [linux]
 [group('vr')]
 vr:
   systemctl --user start wivrn.service
-  systemctl --user stop wivrn-audio.service | complete | ignore
-  systemd-run --user --collect --unit=wivrn-audio nu {{absolute_path("scripts/wivrn-audio.nu")}} switch --timeout 600
 
-# Route audio to a headset that is connected right now.
-[linux]
-[group('vr')]
-vr-audio:
-  nu {{absolute_path("scripts/wivrn-audio.nu")}} switch --timeout 60
-
-# Stop PC VR when finished and restore the previous audio devices.
+# Stop PC VR when finished.
 [linux]
 [group('vr')]
 vr-stop:
-  systemctl --user stop wivrn-audio.service | complete | ignore
-  nu {{absolute_path("scripts/wivrn-audio.nu")}} restore
   systemctl --user stop wivrn.service
 
 # Show PC VR service status.