T14-01 Implement Substack live API drift canary

[edithatogo]

Parent: #465

Implement the T14-01 scheduled live drift canary, deterministic tests, redacted receipts, operator setup documentation, and Slack/Discord/PagerDuty alerts.

Acceptance requires read-only probes, fail-closed configuration, six-hour scheduling, no secret leakage, green required Actions, reviewed comments, and merge/cleanup receipt.

[edithatogo]

Reopened conservatively. Archived implementation evidence remains valid; #532 is the verification continuation required before this track closes.