ci(codeql): CodeQL cron weekly → monthly (#34)

Retires the orphan cicd/codeql-cron-monthly branch by re-applying its single change (CodeQL scheduled scan weekly → monthly, standards#288) onto current main.

Author: hyperpolymath

.github/workflows/codeql.yml

@@ -7,7 +7,7 @@ on:
   pull_request:
     branches: [main, master]
   schedule:
-    - cron: '0 6 * * 1'
+    - cron: '0 6 1 * *'   # monthly 1st 06:00 UTC
 
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   contents: read