[FEATURE] Fire hooks on background task completion

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single feature request (not multiple bundled together)

What do you want?

No hooks fire on background task completion (run_in_background: true). As a result, it is not possible to use hooks to add additional context when background tasks complete.

Background task completions and Monitor notifications are injected directly into the conversation, bypassing the hook pipeline entirely. Either PostToolUse should fire when a background task completes, or a new hook event type should be added for this.