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/providers/inline/files/localfs/files.py

@@ -46,7 +46,7 @@ class LocalfsFilesImpl(Files):
         storage_path.mkdir(parents=True, exist_ok=True)
 
         # Initialize SQL store for metadata
-        self.sql_store = AuthorizedSqlStore(sqlstore_impl(self.config.metadata_store), self.policy)
+        self.sql_store = AuthorizedSqlStore(sqlstore_impl(self.config.persistence), self.policy)
         await self.sql_store.create_table(
             "openai_files",
             {