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/postgres-demo/run.yaml

@@ -73,7 +73,14 @@ providers:
     provider_type: remote::model-context-protocol
 persistence:
   backends:
-    default:
+    kvstore:
+      type: postgres
+      host: ${env.POSTGRES_HOST:=localhost}
+      port: ${env.POSTGRES_PORT:=5432}
+      db: ${env.POSTGRES_DB:=llamastack}
+      user: ${env.POSTGRES_USER:=llamastack}
+      password: ${env.POSTGRES_PASSWORD:=llamastack}
+    sqlstore:
       type: postgres
       host: ${env.POSTGRES_HOST:=localhost}
       port: ${env.POSTGRES_PORT:=5432}
@@ -82,9 +89,9 @@ persistence:
       password: ${env.POSTGRES_PASSWORD:=llamastack}
   stores:
     metadata:
-      backend: default
+      backend: kvstore
     inference:
-      backend: default
+      backend: sqlstore
 models:
 - metadata: {}
   model_id: ${env.INFERENCE_MODEL}