Defer model-usage and anthropic-manager from the buildable set (issue 060)

Both carry third-party runtime deps (pg; tweetnacl + sealedbox) that
their Dockerfile installs with npm — which 404s in a mesh build, whose
npm points at the mesh's own registry, not public npm. The workstation
build script got away with it by installing on a host with public npm.
Delivering a module's third-party deps into a mesh build is an open
question (how: publish to the mesh registry, or proxy); until it is
answered these two stay on the placeholder path they were already on.
The other 37 modules build from their own directory with no external
fetch.

modules/model-usage/module.json

@@ -49,6 +49,7 @@
       "id": "runtime",
       "type": "container",
       "name": "mesh-model-usage",
+      "image": "mesh-runtime-model-usage@sha256:0000000000000000000000000000000000000000000000000000000000000000",
       "network": "host",
       "volumes": [
         "/var/lib/mesh/model-usage/broker:/run/secrets/broker:ro",
@@ -59,29 +60,7 @@
       },
       "env-file": [
         "/var/lib/model-usage/db.env"
-      ],
-      "artifact": "runtime"
+      ]
     }
-  ],
-  "build": {
-    "on": [
-      {
-        "arg": "BUILD_BASE",
-        "module": "mesh-tools",
-        "artifact": "build"
-      },
-      {
-        "arg": "RUNTIME_BASE",
-        "module": "mesh-tools",
-        "artifact": "runtime"
-      }
-    ],
-    "artifacts": [
-      {
-        "name": "runtime",
-        "kind": "image",
-        "from": "Dockerfile"
-      }
-    ]
-  }
+  ]
 }