boot_preflight: skip /dev/tty fallback without a tty (#225)

scripts/boot_host_preflight.sh

@@ -82,6 +82,9 @@ RESEARCH_GUEST_STATUS="$(
       fi
     fi
     # No Macintosh HD found, or auto-select failed — prompt interactively
+    # only when a controlling TTY exists. Non-interactive runs should degrade
+    # cleanly instead of failing on /dev/tty access under set -e.
+    if { : >/dev/tty; } 2>/dev/null; then
       printf '%s\n' "$_out" >/dev/tty
       printf 'Pick a macOS installation: ' >/dev/tty
       read _choice </dev/tty
@@ -91,6 +94,9 @@ RESEARCH_GUEST_STATUS="$(
       else
         echo 'unavailable'
       fi
+    else
+      echo 'unavailable (multiple installs; no interactive tty)'
+    fi
   fi
 )"
 CURRENT_BOOT_ARGS="$(sysctl -n kern.bootargs 2>/dev/null || true)"