ci(codeql): cron weekly→monthly (cut 3, standards#288) (#68)

Author: hyperpolymath

.github/workflows/codeql.yml

@@ -6,7 +6,8 @@ on:
   pull_request:
     branches: [main, master]
   schedule:
-    - cron: '0 6 * * 1'
+    - cron: '0 6 1 * *'   # monthly 1st 06:00 UTC
+
 # Estate guardrail: cancel superseded runs so re-pushes / rebased PR
 # updates do not pile up queued runs against the shared account-wide
 # Actions concurrency pool. Applied only to read-only check workflows