feat(context-injector): implement messages transform hook for context injection

- Implement `createContextInjectorMessagesTransformHook` for messages transform hook
- Refactor existing `chat.message` handler to be a no-op (context injection moved to transform)
- Add comprehensive test suite for the new hook (4 test cases)
- Update exports to expose new hook function

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/context-injector/index.ts

@@ -1,5 +1,9 @@
 export { ContextCollector, contextCollector } from "./collector"
-export { injectPendingContext, createContextInjectorHook } from "./injector"
+export {
+  injectPendingContext,
+  createContextInjectorHook,
+  createContextInjectorMessagesTransformHook,
+} from "./injector"
 export type {
   ContextSourceType,
   ContextPriority,