feat(config): allow runtime_fallback to be configured as boolean

Enable simple boolean configuration for runtime_fallback:
- "runtime_fallback": true - Enable with defaults
- "runtime_fallback": false - Disable
- "runtime_fallback": { ... } - Advanced object config (existing)

Updated schema, event handler, chat-message handler, and session hooks
to handle both boolean and object formats.

src/plugin/chat-message.ts

@@ -62,7 +62,9 @@ export function createChatMessageHandler(args: {
   const isRuntimeFallbackEnabled =
     hooks.runtimeFallback !== null &&
     hooks.runtimeFallback !== undefined &&
-    (pluginConfig.runtime_fallback?.enabled ?? true)
+    (typeof pluginConfig.runtime_fallback === "boolean"
+      ? pluginConfig.runtime_fallback
+      : (pluginConfig.runtime_fallback?.enabled ?? true))
 
   return async (
     input: ChatMessageInput,