Enable Early Hints debug mode to verify if Rails sends HTTP 103

This temporarily enables debug mode in shakapacker to output HTML comments
showing whether Rails is attempting to send HTTP 103 Early Hints responses.

This will help diagnose if the issue is:
- Rails not sending 103 at all
- Thruster not forwarding 103 responses
- Control Plane load balancer stripping 103 responses

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

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

Author: justin808

config/shakapacker.yml

@@ -67,4 +67,4 @@ production:
   # Early hints configuration
   early_hints:
     enabled: true
-    debug: false # Set to true to output debug info as HTML comments
+    debug: true # Set to true to output debug info as HTML comments