Add cascade-delete finalizer to stateless apps; document the distinction

docs/08-adding-new-apps.md

@@ -88,6 +88,33 @@ addresses listed in [`docs/network.md`](network.md), and add a row for it
 there once it's up — that file only stays useful if every app that gets an
 address shows up in it.
 
+## Removing an app
+
+The reverse of adding one: delete `apps/<name>.yaml` and `apps/<name>/`,
+commit, and push. The app-of-apps notices the missing Application manifest
+and prunes the `Application` object itself — but that alone does **not**
+delete what that Application deployed. Argo CD only cascades an
+Application's deletion into deleting its managed resources (namespace,
+Deployment, Service, everything) if that Application carries this
+finalizer:
+
+```yaml
+metadata:
+  finalizers:
+    - resources-finalizer.argocd.argoproj.io
+```
+
+Without it, the resources are simply orphaned — still running, but no
+longer tracked by Argo CD or described anywhere in Git. `apps/demo.yaml`
+and `apps/metallb.yaml` both carry this finalizer. `apps/longhorn.yaml`
+deliberately does **not** — see the comment in that file for why cascading
+a storage layer's deletion is a different, much higher-stakes decision than
+cascading a stateless app's.
+
+> **Ask Claude:** "what would happen right now if I deleted
+> `apps/longhorn.yaml` and it had this finalizer?" before you ever add one
+> to an app that holds real data.
+
 ## Checklist
 
 - [ ] New app's manifests live in `apps/<name>/`