Offer OEM mode only when the bundled runtime supports it

The configurator now peeks into the offline mirror's runtime package for
omarchy-oem-setup before offering the OEM install mode, so an ISO built
against a pre-OEM runtime can't start an install doomed to fail at the
staging phase (which still hard-checks as the backstop). Blank disks with no
OEM choice to offer skip the mode picker entirely, like the pre-OEM flow.

Also keep the test harness QMP socket in /tmp — unix socket paths cap at
~108 bytes, which nested test-run directories can exceed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/omarchy-iso-test

@@ -101,7 +101,9 @@ HTTP_PID=""
 
 mkdir -p "$BASE_DIR" "$RUN_DIR"
 
-QMP_SOCK="$RUN_DIR/qmp.sock"
+# Unix socket paths are capped at ~108 bytes, which a run directory nested in
+# test-runs/<iso-name>/runs/<timestamp> can exceed. Keep the socket in /tmp.
+QMP_SOCK=$(mktemp -u "${TMPDIR:-/tmp}/omarchy-iso-test-qmp.XXXXXX.sock")
 PIDFILE="$RUN_DIR/qemu.pid"
 
 log() {