Python Service: Schema Drift Gate + Repair Path

Schema-dependent services must gate drift and provide deterministic repair.

Rationale:
- Drift between committed schema artifacts and upstream contracts can silently break generated clients.

Scope:
- PR review policy for Python services with schema snapshot + codegen toolchain.

Trigger:
- A PR adds or changes schema drift detection, snapshot refresh, or generation workflows.

Approval checks:
1) A drift-check command exists and fails on detected drift or failed contract retrieval.
2) A refresh/regenerate command exists for deterministic reconciliation.
3) Required CI gates run drift/contract checks before publish/deploy jobs.
4) README/runbook documents local remediation steps.

Evidence:
- Script/workflow/docs diffs show all four controls.

Exception path:
- Allow deviation only when all are true:
1) PR description includes `Policy Exception: python-schema-drift-and-repair`.
2) PR explains why drift cannot be CI-gated in normal flow.
3) PR documents alternative contract assurance.

Decision:
- Return NOT APPROVED when trigger is met and drift gate/repair controls are missing without a valid exception.