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).

Makefile

@@ -80,6 +80,12 @@ test-dev:
 	sh infra/local/tools.sh test
 	sh infra/local/tools.sh verify
 
+# Production pins (infra/prod/README.md): resolved once, copied to every node.
+.PHONY: prod-images
+prod-images:
+	sh infra/local/tools.sh build
+	sh infra/local/tools.sh prod-images
+
 .PHONY: test-dev-infra
 test-dev-infra:
 	sh infra/local/tools.sh test