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/index.ts

@@ -20,6 +20,7 @@ import {
   createAgentUsageReminderHook,
   createNonInteractiveEnvHook,
   createInteractiveBashSessionHook,
+  createEmptyMessageSanitizerHook,
 } from "./hooks";
 import { createGoogleAntigravityAuthPlugin } from "./auth/antigravity";
 import {
@@ -246,6 +247,9 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
   const interactiveBashSession = isHookEnabled("interactive-bash-session")
     ? createInteractiveBashSessionHook(ctx)
     : null;
+  const emptyMessageSanitizer = isHookEnabled("empty-message-sanitizer")
+    ? createEmptyMessageSanitizerHook()
+    : null;
 
   updateTerminalTitle({ sessionId: "main" });
 
@@ -281,6 +285,14 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
       await keywordDetector?.["chat.message"]?.(input, output);
     },
 
+    "experimental.chat.messages.transform": async (
+      input: Record<string, never>,
+      output: { messages: Array<{ info: unknown; parts: unknown[] }> }
+    ) => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      await emptyMessageSanitizer?.["experimental.chat.messages.transform"]?.(input, output as any);
+    },
+
     config: async (config) => {
       const builtinAgents = createBuiltinAgents(
         pluginConfig.disabled_agents,