[Accessibility] Agent count announcement remains stale after deleting a workspace agent

## Type of issue

Accessibility

## Description

Deleting a workspace custom agent updates the visible Agent Customizations list and Chat agent picker, but the shared ARIA live status retains the previous agent count. Screen-reader users can therefore receive an incorrect result count after the inventory changes.

## Steps to reproduce

1. Open a trusted workspace containing two `.github/agents/*.agent.md` files.
2. Select the Copilot harness and open **Customizations > Agents**.
3. Create and save a third workspace agent.
4. Delete one workspace agent through its **More Actions** menu and accept the confirmation.
5. Open the Chat custom-agent picker and inspect or listen to the live status text.

## Expected behavior

The picker contains two agents and the ARIA live status reports `2 agents`.

## Actual behavior

The visible Customizations list and Chat picker correctly contain only the two surviving agents, but the live status remains:

```html

## Environment

- VS Code Insiders: `1.138.0-insider`
- Commit: `7debcd0e2acdea1c52de81bf9ee1620444407dda`
- OS: Windows

Originally found while testing aeschli/agent-testing#2.