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/prod/vault/config/vault-3.hcl

@@ -0,0 +1,30 @@
+# SXCHNGE production Vault, node 3. Only api_addr, cluster_addr and node_id
+# differ between the three node files.
+ui            = false
+disable_mlock = false
+api_addr      = "http://192.168.10.98:8200"
+cluster_addr  = "https://192.168.10.98:8201"
+
+listener "tcp" {
+  address         = "0.0.0.0:8200"
+  cluster_address = "0.0.0.0:8201"
+  # Plain HTTP on the private network; raft and request forwarding on the
+  # cluster port are encrypted by Vault itself.
+  tls_disable = true
+}
+
+storage "raft" {
+  path    = "/vault/file"
+  node_id = "sxchnge-vault-3"
+  retry_join { leader_api_addr = "http://192.168.10.191:8200" }
+  retry_join { leader_api_addr = "http://192.168.10.191:8210" }
+  retry_join { leader_api_addr = "http://192.168.10.98:8200" }
+}
+
+# Auto-unseal with the platform Vault. The token comes from VAULT_TOKEN, set by
+# start.sh from the secret file; recovery keys stay with people.
+seal "transit" {
+  address    = "https://vault.sendico.io"
+  mount_path = "transit/"
+  key_name   = "sxchnge-vault-unseal"
+}