[eric] apps: the scaffold ships a durable store and the skill forbids memory-only state, since the runtime dies on a timer

backend/apps/outputs/app_builder_skill.md

@@ -274,6 +274,22 @@ and flips `BACKEND_PORT` in both `.env` and `.env.example`. Then run
 - Install your own venv or `pip install` manually.
 - Edit `backend/run.sh` or the SubApp framework.
 
+**Persist anything the user comes back to. Your process is disposable.**
+OpenSwarm freezes this app's process when its card closes and fully kills
+it after ~15 minutes idle, on quit, and on crash. A module-level list or
+dict is therefore data loss on a timer. The scaffold ships a durable
+store; use it (or your own files under `backend/data/`):
+
+```python
+from backend.apps.store.store import load_store, save_store
+
+data = load_store()              # {} on first run, never raises
+data["items"] = [*data.get("items", []), new_item]
+save_store(data)                 # atomic write; a kill mid-write keeps the old data
+```
+
+Holding state only in memory is a bug, not a style choice.
+
 Adding a new endpoint is just adding a new SubApp:
 
 ```python

backend/apps/outputs/webapp_template/backend/apps/store/store.py

@@ -0,0 +1,52 @@
+"""Disk-backed app state. Use this instead of module-level variables for anything worth keeping.
+
+The app's process is DISPOSABLE: OpenSwarm freezes it when its card closes, kills it after ~15
+minutes idle, on quit, and on crash. A module-level list or dict therefore silently loses the
+user's data on a schedule you don't control. This store survives all of that: one JSON file under
+backend/data/, written atomically so a kill mid-write can never corrupt it.
+
+    from backend.apps.store.store import load_store, save_store
+
+    items = load_store().get("items", [])
+    items.append(new_item)
+    save_store({**load_store(), "items": items})
+"""
+
+import json
+import os
+import tempfile
+from typing import Any, Dict
+
+from typeguard import typechecked
+
+P_BACKEND_DIR = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+DATA_DIR = os.path.join(P_BACKEND_DIR, "data")
+STORE_PATH = os.path.join(DATA_DIR, "store.json")
+
+
+@typechecked
+def load_store() -> Dict[str, Any]:
+    """The whole store as a dict; empty on first run or an unreadable file, never an exception."""
+    try:
+        with open(STORE_PATH, "r", encoding="utf-8") as f:
+            data = json.load(f)
+        return data if isinstance(data, dict) else {}
+    except (FileNotFoundError, json.JSONDecodeError, OSError):
+        return {}
+
+
+@typechecked
+def save_store(data: Dict[str, Any]) -> None:
+    """Replace the store atomically: temp file then rename, so a kill mid-write leaves the old data."""
+    os.makedirs(DATA_DIR, exist_ok=True)
+    fd, tmp = tempfile.mkstemp(dir=DATA_DIR, suffix=".tmp")
+    try:
+        with os.fdopen(fd, "w", encoding="utf-8") as f:
+            json.dump(data, f, ensure_ascii=False, indent=1)
+        os.replace(tmp, STORE_PATH)
+    except OSError:
+        try:
+            os.unlink(tmp)
+        except OSError:
+            pass
+        raise