Consolidates the scattered infra facts (docker host layout, driver fixtures, the
three env-gated live-gate recipes, the Host.IntegrationTests harness, the HistGW
local deploy, docker-dev rig, shared GLAuth, deploy) into infra/README.md as the
single entry point, cross-linked to dev-environment.md + the deep docs. CLAUDE.md
points at it from the Docker Workflow section.
Joseph Doherty