fix(observability-client): 🐛 fix remote write URL and add missing manifests dir

- Fix broken remote write URL: o12y.observability. → o12y.observability.buildth.ing
- Create manifests/ directory with .gitkeep for ArgoCD source path

otc/dev.t09.de/stacks/observability-client/vm-client-stack/values.yaml

@@ -778,7 +778,7 @@ vmagent:
   # -- Remote write configuration of VMAgent, allowed parameters defined in a [spec](https://docs.victoriametrics.com/operator/api#vmagentremotewritespec)
   additionalRemoteWrites:
     # []
-    - url: https://o12y.observability./api/v1/write
+    - url: https://o12y.observability.buildth.ing/api/v1/write
       basicAuth:
         username:
           name: simple-user-secret