Run the production Vault as three raft nodes on exch and infra-gamma

Two nodes on exch and one on infra-gamma, plain Compose per server, ports
on the private addresses only. Nodes unseal through the transit seal on the
platform Vault; the transit token reaches only the server process.

make prod-images resolves the latest stable Vault release once and pins its
digest in ignored .local/prod/images.env, so every node runs the same build.

infra/prod/README.md records the layout, the first rollout, node
replacement and recovery when the platform Vault is lost (#74).

infra/local/scripts/dispatch.sh

@@ -5,6 +5,7 @@ set -o pipefail
 . infra/local/scripts/secrets.sh
 case "${1:-}" in
  images) . infra/local/scripts/images.sh; images;;
+ prod-images) TOOL_STATE="$PWD/.local"; . infra/local/scripts/images.sh; prod_images;;
  up) . infra/local/scripts/bootstrap.sh; up;;
  email) . infra/local/scripts/bootstrap.sh; email;;
  sendico) . infra/local/scripts/bootstrap.sh; . infra/local/scripts/sendico.sh; configure_sendico;;