Declare every module's data; the backup holder measures it (hq ADR 0233)

Backup lines are derived from each module's data section instead of written by hand; the holder
measures declared items, reads the array under them, and deletes a retired item only after a last
restore point; the Go providers say each held consumer's size so an empty replacement is seen.

modules/ssh-client/module.json

@@ -11,6 +11,16 @@
     "ssh_client_known_host",
     "ssh_client_test"
   ],
+  "data": {
+    "own": [
+      {
+        "id": "ssh",
+        "path": "${dir:ssh-dir}",
+        "class": "valuable",
+        "why": "the operator's keys and known hosts"
+      }
+    ]
+  },
   "resources": [
     {
       "id": "ssh-dir",