feat: usb-automount feature — udev+systemd auto-mount of USB sticks, flag-gated like autostart

features/usb-automount.sh (installed via ./install.sh --feature, flag
usb-automount) mounts every unmounted removable block device at
/media/<label> — world-writable via -o umask=000 (fallback plain mount),
label-collision bump -2/-3, no-label -> usb-<name>. First root run
self-installs the hotplug udev rule (/etc/udev/rules.d/99-usb-automount.rules,
SYSTEMD_WANTS=usb-automount.service) + udevadm reload/trigger; an existing
rule is never overwritten. systemd/usb-automount.service (Type=oneshot,
WantedBy=multi-user.target) covers boot + hotplug + manual start, gated in
postinstall.sh's systemd loop exactly like autostart. Purpose: a plugged-in
stick is ready for pos system backup's post-verify USB copy without manual
mounting. Docs: SYSTEMD/SCRIPTS/README/AGENT_Context (tree, filetable —
postinstall.sh count corrected 152->163, it was already stale). Verified:
stub suite /tmp/opencode/usb-automount-test 47/47 green; make gen && make check.

postinstall.sh

@@ -147,6 +147,11 @@ if [ -d systemd ] && [ -n "$(ls -A systemd/*.service 2>/dev/null)" ]; then
             warn "autostart feature not installed — skipping autostart.service (run ./install.sh --feature)"
             continue
         fi
+        # usb-automount.service runs features/usb-automount.sh — same gating
+        if [ "$svc_name" = "usb-automount.service" ] && ! flag_is_set usb-automount; then
+            warn "usb-automount feature not installed — skipping usb-automount.service (run ./install.sh --feature)"
+            continue
+        fi
         run sudo systemctl enable --now "$svc_name" 2>/dev/null || \
             run sudo systemctl enable "$svc_name"
     done