ci(preflight): enforce ops coverage (EXPLAIN + pgTAP) behind toggle #293
Description
Context\n- Preflight already validates QIR/Envelope/Manifest. Add an optional toggle to require EXPLAIN snapshots and pgTAP presence for compiled ops.\n\nAcceptance\n- Toggle env/flag causes preflight to exit non‑zero when ops coverage is incomplete\n- Docs explain how to enable it\n\nLabels: ci, tests, group:qir-phase-c