Simplify to single 'default' backend name

- Replace kvstore/sqlstore backend names with unified 'default'
- Context-aware parsing handles KVStore vs SqlStore based on usage
- Provider configs use 'backend: default' everywhere
- Much cleaner: kvstore { backend: default } instead of { backend: kvstore }

llama_stack/distributions/starter/run.yaml

@@ -220,17 +220,14 @@ providers:
         db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/batches.db
 persistence:
   backends:
-    kvstore:
+    default:
       type: sqlite
-      db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/kvstore.db
-    sqlstore:
-      type: sqlite
-      db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/sqlstore.db
+      db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/store.db
   stores:
     metadata:
-      backend: kvstore
+      backend: default
     inference:
-      backend: sqlstore
+      backend: default
 models: []
 shields:
 - shield_id: llama-guard