feat(audio): prefer dock/HDMI/USB sinks in WirePlumber (#87)

Raise priority.session for HDMI/DisplayPort (1100) and USB (1050) ALSA
outputs so the default sink follows dock plug/unplug; built-in analog
wins again when externals disappear. Rules live in
modules/nixos/dock-audio-rules.nix; checks.dock-audio covers the contract.

Verified: V2 — flake check --no-build; checks.dock-audio green.
V3 pending: HARDWARE-QUEUE #87 real HDMI/USB-dock hotplug.

agent/HARDWARE-QUEUE.md

@@ -19,6 +19,13 @@ the **T14s** (webcam case).
       — should suspend as usual. Optional: with a docked display-profile
       (`eDP-1` disable), confirm clamshell layout still looks right after
       lid close. Pass = no surprise suspend when docked.
+- [ ] **#87 dock audio sink follow** — V2 asserts WirePlumber
+      `90-nomarchy-dock-audio` rules (HDMI/USB priority). On a laptop
+      with HDMI or USB-dock audio: play sound on speakers, plug external
+      audio → default sink should move to HDMI/dock (`wpctl status` shows
+      `*` on the external sink); unplug → `*` returns to built-in.
+      Manual `wpctl set-default` still works; Bluetooth may outrank dock
+      (by design). Pass = auto switch without menu intervention.
 - [ ] **#55 fingerprint enroll on real reader** — with
       `nomarchy.hardware.fingerprint.enable` and a physical reader: System ›
       Fingerprint › Enroll a finger; List shows it; Verify succeeds; optional