Runtime config: dedicated mergeable file + restart-on so settings take effect (issue 009)

Give each settings-config runtime a mergeable config file with its own id
(runtime-config) — the previous "config" collided with modules that already own a
config directory, so ten runtimes mounted a config file no resource declared. Point
each runtime's restart-on at it, so a settings change recreates the runtime and it
re-reads the new value (needs the mesh-host container restart-on fix on
issue/009-container-restart-on). Proven: runtime-restart-on-config e2e green.

Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF

modules/nextcloud/module.json

@@ -83,7 +83,7 @@
       ]
     },
     {
-      "id": "config",
+      "id": "runtime-config",
       "type": "file",
       "path": "/var/lib/mesh/nextcloud/config.json",
       "mode": "0600",
@@ -105,7 +105,10 @@
         "MESH_BROKER_FILE": "/run/secrets/broker",
         "MESH_NEXTCLOUD_URL": "http://127.0.0.1:80",
         "MESH_NEXTCLOUD_CONFIG_FILE": "/run/config/config.json"
-      }
+      },
+      "restart-on": [
+        "runtime-config"
+      ]
     }
   ]
 }