chore(ai): retire AIAuditBridge Codex Audit dispatch

[Pigbibi]

Removes the monthly dispatch to the retired AIAuditBridge codex_audit.yml workflow (v1 Codex Audit is retired; dispatches 404 or no-op). The monthly issue / artifact is still created for human review; no replacement AI call is added.

• Delete the GitHub App token, CODEX_AUDIT_* env/vars usage and dispatch steps.
• Update synthetic workflow-config tests to assert the retired bridge cannot return.
• No secrets read or created, no deployment, no production rerun. Owner may delete the now-unused CODEX_AUDIT_DISPATCH_TOKEN secret and CODEX_AUDIT_* variables after merge.