Add option to suppress async hook completion notifications

Problem

When multiple plugins are enabled, each tool call triggers numerous Async hook PreToolUse completed and Async hook PostToolUse completed notification messages in the terminal output. With 20+ plugins enabled, this creates significant visual noise that makes it harder to follow the actual conversation.

Example output:

⎿  Async hook PostToolUse completed
⎿  Async hook PreToolUse completed
⎿  Async hook PostToolUse completed
⎿  Async hook PreToolUse completed
⎿  Async hook PostToolUse completed
⎿  Async hook PreToolUse completed
⎿  Async hook PostToolUse completed
⎿  Async hook PreToolUse completed
⎿  Async hook PreToolUse completed
⎿  Async hook PreToolUse completed
⎿  Async hook PostToolUse completed

Proposed Solution

Add a setting in settings.json (or settings.local.json) to control the visibility of async hook completion notifications. For example:

{
  "showAsyncHookNotifications": false
}

Or a more granular option:

{
  "hookNotifications": {
    "showAsyncCompletion": false
  }
}

Context

• These messages come from plugin-registered async hooks (PreToolUse/PostToolUse)
• The hooks directory may be empty — all notifications originate from enabled plugins
• There is currently no way to suppress these messages without disabling the plugins themselves
• Users with many plugins enabled are most affected

Alternatives Considered

• Disabling plugins: Not ideal since users want plugin functionality, just less visual noise
• Only showing notifications for user-defined hooks (not plugin hooks)