fix(hook-message-injector): add validation to prevent empty message injection and improve logging

- Add content validation in injectHookMessage() to prevent empty hook content injection
- Add logging to claude-code-hooks and keyword-detector for better debugging
- Document timing issues in empty-message-sanitizer comments
- Update README with improved setup instructions

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

src/hooks/empty-message-sanitizer/index.ts

@@ -8,6 +8,12 @@ interface MessageWithParts {
 }
 
 type MessagesTransformHook = {
+// NOTE: This sanitizer runs on experimental.chat.messages.transform hook,
+// which executes AFTER chat.message hooks. Filesystem-injected messages
+// from hooks like claude-code-hooks and keyword-detector may bypass this
+// sanitizer if they inject empty content. Validation should be done at
+// injection time in injectHookMessage().
+
   "experimental.chat.messages.transform"?: (
     input: Record<string, never>,
     output: { messages: MessageWithParts[] }