feat(hooks): add empty-message-sanitizer to prevent API errors from empty chat messages

Add new hook that uses the `experimental.chat.messages.transform` hook to prevent 'non-empty content' API errors by injecting placeholder text into empty messages BEFORE they're sent to the API.

This is a preventive fix - unlike session-recovery which fixes errors after they occur, this hook prevents the error from happening by sanitizing messages before API transmission.

Files:
- src/hooks/empty-message-sanitizer/index.ts (new hook implementation)
- src/hooks/index.ts (export hook function)
- src/config/schema.ts (add hook to HookName type)
- src/index.ts (wire up hook to plugin)

🤖 Generated with assistance of OhMyOpenCode (https://github.com/code-yeongyu/oh-my-opencode)

src/hooks/index.ts

@@ -19,3 +19,4 @@ export { createAgentUsageReminderHook } from "./agent-usage-reminder";
 export { createKeywordDetectorHook } from "./keyword-detector";
 export { createNonInteractiveEnvHook } from "./non-interactive-env";
 export { createInteractiveBashSessionHook } from "./interactive-bash-session";
+export { createEmptyMessageSanitizerHook } from "./empty-message-sanitizer";