feat!: providers use unified 'persistence' field

BREAKING CHANGE: Provider config field names changed for semantic clarity

- Rename kvstore → persistence for all providers
- Simple providers: flat persistence with backend reference
- Complex providers (agents): nested persistence.agent_state + persistence.responses
- Files provider: metadata_store → persistence
- Provider configs now clearly express 'how do I persist?' not 'what type of store?'

Example:
  # Before
  config:
    kvstore:
      backend: kvstore
      namespace: faiss

  # After
  config:
    persistence:
      backend: kvstore
      namespace: faiss

  # Agents (nested)
  config:
    persistence:
      agent_state:
        backend: kvstore
        namespace: agents
      responses:
        backend: sqlstore
        namespace: responses

llama_stack/distributions/starter/run.yaml

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