fix(model-fallback): keep model fallback opt-in by default

Restore the runtime default that was introduced for model fallback so unset config no longer enables automatic retries unexpectedly.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin/hooks/create-session-hooks.ts

@@ -153,7 +153,7 @@ export function createSessionHooks(args: {
 
   // Model fallback hook (configurable via model_fallback config + disabled_hooks)
   // This handles automatic model switching when model errors occur
-  const isModelFallbackConfigEnabled = pluginConfig.model_fallback ?? true
+  const isModelFallbackConfigEnabled = pluginConfig.model_fallback ?? false
   const modelFallback = isModelFallbackConfigEnabled && isHookEnabled("model-fallback")
     ? safeHook("model-fallback", () =>
       createModelFallbackHook({