fix: prevent silent exit when registration_token is empty

The [[ -n "$json_token" ]] && ... pattern returns exit code 1
when json_token is empty, causing script to exit due to set -e.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

script/start.sh

@@ -142,7 +142,7 @@ parse_json() {
 
     # Registration token (can also be set via environment variable)
     local json_token=$(echo "$json" | jq -r '.registration_token // empty')
-    [[ -n "$json_token" ]] && SYSADMIN_REGISTRATION_TOKEN="$json_token"
+    [[ -n "$json_token" ]] && SYSADMIN_REGISTRATION_TOKEN="$json_token" || true
 }
 
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