chore(ci): replace secret-scanner.yml with reusable wrapper (#20)

Pins to hyperpolymath/standards#190 merge SHA 3e4bd4c93911750727e2e4c66dff859e00079da0. Force-propagates
the shell-secrets job (added post-Cloudflare-leak 2026-05-21) to this
repo's secret-scanning gate. Replaces ~75-116 lines with a ~14-line wrapper.

Part of estate-wide convergence campaign 2026-05-26
(standards#199 / #190).

Author: hyperpolymath