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.test.ts

@@ -0,0 +1,56 @@
+import { describe, expect, it } from "bun:test"
+import type { OhMyOpenCodeConfig } from "../../config"
+import type { ModelCacheState } from "../../plugin-state"
+import type { PluginContext } from "../types"
+import { createSessionHooks } from "./create-session-hooks"
+
+const mockContext = {
+  directory: "/tmp",
+  client: {
+    tui: {
+      showToast: async () => ({}),
+    },
+    session: {
+      get: async () => ({ data: null }),
+      update: async () => ({}),
+    },
+  },
+} as unknown as PluginContext
+
+const mockModelCacheState = {} as ModelCacheState
+
+describe("createSessionHooks", () => {
+  it("keeps model fallback disabled when config is unset", () => {
+    // given
+    const pluginConfig = {} as OhMyOpenCodeConfig
+
+    // when
+    const result = createSessionHooks({
+      ctx: mockContext,
+      pluginConfig,
+      modelCacheState: mockModelCacheState,
+      isHookEnabled: (hookName) => hookName === "model-fallback",
+      safeHookEnabled: true,
+    })
+
+    // then
+    expect(result.modelFallback).toBeNull()
+  })
+
+  it("creates model fallback hook when config explicitly enables it", () => {
+    // given
+    const pluginConfig = { model_fallback: true } as OhMyOpenCodeConfig
+
+    // when
+    const result = createSessionHooks({
+      ctx: mockContext,
+      pluginConfig,
+      modelCacheState: mockModelCacheState,
+      isHookEnabled: (hookName) => hookName === "model-fallback",
+      safeHookEnabled: true,
+    })
+
+    // then
+    expect(result.modelFallback).not.toBeNull()
+  })
+})

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({