Add merge_group trigger to support GitHub merge queues

Enable octoguard security checks to run on merge queue events.
This ensures the workflow runs when PRs are added to merge queues,
preventing merge queue checks from waiting indefinitely.

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

Author: jganoff

.github/workflows/octoguard.yml

@@ -4,6 +4,7 @@ on:
   workflow_call:
   workflow_dispatch:
   pull_request:
+  merge_group:
   push:
     branches:
       - main