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/build-agent/module.json

@@ -25,6 +25,16 @@
   "own-secrets": {
     "broker": "${dir:mesh-state}/broker"
   },
+  "data": {
+    "own": [
+      {
+        "id": "workspace",
+        "path": "${dir:workspace}",
+        "class": "cache",
+        "why": "a build's working copy, cloned again for every build"
+      }
+    ]
+  },
   "resources": [
     {
       "id": "mesh-state",