fix(debug-system): drop ES export so the file loads as a classic script

Markitect loads js/core/debug-system.js via a plain <script> tag; the trailing
export made it a SyntaxError and window.MarkitectDebugSystem was never set.
Nothing imports the named export; the bundle uses a side-effect import.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: sonnet
Assistant-Process: 299063@bnt-lap001
Assistant-Session: 53eb1e1c-c13a-4277-9ce2-3702960d93f7

js/core/debug-system.js

@@ -289,6 +289,3 @@ class MarkitectDebugSystem {
 
 // Initialize and expose globally
 window.MarkitectDebugSystem = new MarkitectDebugSystem();
-
-// ES6 export for bundler
-export { MarkitectDebugSystem };