chore(ci): replace mirror.yml with reusable wrapper (#18)

Pins to hyperpolymath/standards#187 merge SHA e6b2884722350515934d443daf23442f2195796f. Replaces the
canonical mirror.yml (~145 lines, drift-prone) with a thin ~13-line
wrapper. Forge selection still externalised to vars.<FORGE>_MIRROR_ENABLED.

Part of estate-wide convergence campaign 2026-05-26
(standards#199 / #187).

Author: hyperpolymath