fix(cli): config nucleus blank input no longer clobbers an existing token

The prompt says "leave it blank to skip", but the blank path
unconditionally copied the guest template over omni_pass.env, silently
wiping a saved Nucleus API token and breaking Isaac Sim asset auth.
Blank input now keeps an existing file untouched and only writes the
guest defaults when no file exists yet (first-time setup unchanged).

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

Author: andrewjong

.airstack/modules/config.sh

@@ -71,9 +71,14 @@ function cmd_config_nucleus {
             -e "s/^OMNI_PASS=.*$/OMNI_PASS=$escaped_api_token/" \
             "$OMNI_PASS_SOURCE" > "$OMNI_PASS_DESTINATION"
         log_info "Nucleus login configuration complete"
+    elif [ -f "$OMNI_PASS_DESTINATION" ]; then
+        # Blank input must actually skip: the file may hold a working API
+        # token, and overwriting it with the guest template would silently
+        # break Nucleus auth.
+        log_info "Skipping Nucleus login configuration (keeping existing $OMNI_PASS_DESTINATION)"
     else
         cp "$OMNI_PASS_SOURCE" "$OMNI_PASS_DESTINATION"
-        log_info "Skipping Nucleus login configuration"
+        log_info "Skipping Nucleus login configuration (created guest defaults)"
     fi
 }