ci(codeql): cron weekly→monthly (cut 3, standards#288) (#35)

Per `standards#286` canonical (cut 3, Option B 2026-05-30): convert
CodeQL scheduled run from weekly `0 6 * * 1` to monthly `0 6 1 * *`.
PR-trigger runs unchanged — every PR still gets CodeQL.

Refs `hyperpolymath/standards#288` (campaign).

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Copyright (c) Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
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## Summary

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## Changes

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-

## RSR Quality Checklist

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### Required

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- [ ] Code is formatted (`just fmt` or equivalent)
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`Admitted`, `sorry`)
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### As Applicable

- [ ] `.machine_readable/STATE.a2ml` updated (if project state changed)
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- [ ] `.machine_readable/META.a2ml` updated (if architectural decisions
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- [ ] Documentation updated for user-facing changes
- [ ] `TOPOLOGY.md` updated (if architecture changed)
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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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
 
 # Estate guardrail: cancel superseded runs so re-pushes / rebased PR
 # updates do not pile up queued runs against the shared account-wide