v1.5.2 — Linux portability fix

Fixes the hygiene gate on Linux: GNU stat -f reports filesystem info instead of failing, which broke the BSD-first mtime fallback exactly when a scan marker existed. All mtime reads now use python3 (portable, TZ-free). Found by CI's first real run on ubuntu; verified green before this release. Supersedes v1.5.1, whose artifacts carry the broken gate.

Checked: full RELEASE-CHECKLIST AUTO set (bundle refused until closeout/scan/release-test all passed), CI green on ubuntu-latest. Not checked: live claude -p discovery (ran at v1.5.0, skills unchanged since).