Let me control notifications from child sessions

### Feature summary

Add separate notification controls for top-level sessions and their child sessions.

### What problem are you trying to solve?

I often use one main session to start several child sessions. Each child that finishes or asks a question can create its own notification.

With many sessions running, the notifications become constant and important notifications from the main session are easy to miss.

### Proposed solution

Add a notification scope, either globally or for each parent session:

- All sessions
- Top-level sessions only
- Muted

When “Top-level sessions only” is selected, child-session status should remain visible in the app, but individual child sessions should not produce separate OS notifications.

If a child needs input that the parent session cannot handle, the parent could surface that question to the user.

### Workflow impact

This would make parallel orchestration quieter and easier to manage without hiding child-session progress or important questions.

### Installation context

GitHub Copilot app 1.0.23 on Windows x64, OS build 26200.8655, using a main session with multiple child sessions.

### Additional context

Related: #1898 requests separate sound controls for automations, and #248 requests parent-child session organization. Neither covers notification control for child sessions.

This is separate from #1042, which concerns automatic focus switching.

No files, tests, or entry points are named in the issue, so first map the app's notification handling and parent-child session model. Clarify whether scope is global or per parent session, then verify that top-level notifications remain visible, child progress stays in the app, and muted mode suppresses OS notifications.

索引モデルが issue の本文から書いたものです。