ssh-client module: the mesh owns ~/.ssh, config from the hub (to-be 29)

Requires openssh; creates ~/.ssh (0700, owned by the operator account via
${machine:account}); writes every other node's Host block (HostName + User
<account>) into a marked region of ~/.ssh/config (home-scoped, into:block), so
`ssh <node>` reaches each peer as the right account and the operator's own
config is kept. Universal-tier: assigned wherever a person logs in; a node with
no account gets no config.

modules/ssh-client/module.json

@@ -0,0 +1,26 @@
+{
+  "module": "ssh-client",
+  "version": "1",
+  "resources": [
+    {
+      "id": "openssh",
+      "type": "package",
+      "package": "openssh"
+    },
+    {
+      "id": "ssh-dir",
+      "type": "directory",
+      "path": "${machine:account-home}/.ssh",
+      "owner": "${machine:account}",
+      "mode": "0700"
+    }
+  ],
+  "facts": {
+    "ssh-config": {
+      "path": ".ssh/config",
+      "home": true,
+      "shared": true,
+      "template": "# The mesh's Host blocks — every other node, so `ssh <node>` reaches it as the\n# right account. This region is replaced whenever a node joins, leaves or is\n# renamed; the rest of this file is yours and is kept untouched.\n{{range .Machines}}{{if ne .Name $.Node}}\nHost {{.Name}} {{.FQDN}}\n  HostName {{.FQDN}}\n{{if .Account}}  User {{.Account}}\n{{end}}{{end}}{{end}}"
+    }
+  }
+}