Add quiet/silent option for async hooks

Feature Request

When using async hooks (e.g., Stop, UserPromptSubmit), Claude Code displays completion messages like:

• Async hook Stop completed
• Async hook UserPromptSubmit completed

These messages clutter the UI, especially for hooks that run frequently (like TTS or cleanup hooks that fire on every response/prompt).

Proposed Solution

Add a quiet or silent option to hook configuration to suppress the completion message:

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/hook.sh",
            "async": true,
            "quiet": true
          }
        ]
      }
    ]
  }
}

Use Case

I'm using a Stop hook to speak responses via Kokoro TTS, and a UserPromptSubmit hook to stop audio playback. Both work great, but the completion messages after every interaction are noisy and distracting.