fix: os.mjs — show build variant in telemetry, hide CL when unavailable
Build telemetry header now shows "Building (C)" or "Building (Common Lisp)".
CL variant selector only appears once the CL ISO exists on CDN.
Probes cl-native-notepat ISO on boot to determine availability.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>