[FEATURE] Add attention_reason to Notification hooks

Question

### Problem

Determining if a session needs user attention requires parsing the message field with regex. notification_type alone isn't reliable.

### Proposal

Add structured fields:
```json
{
  "hook_event_type": "Notification",
  "attention_reason": "permission_request",
  "requires_user_attention": true
}
```

Enum values: permission_request, needs_input, error, completed, info

### Why

This would make external tools more reliable. Example:
https://github.com/usedhonda/cc-status-bar