[Agents window] /clear command no longer closes session and opens a new one

## Description

In the Agents window, typing `/clear` in the chat input no longer works as expected. Previously, the command would close the current session (marking it as **Done**) and open a new, empty chat session. Now the command does nothing — the session remains open and no new session is created.

The same command works correctly in the regular **Chat view** (sidebar panel), where it does clear the context as expected. The regression is specific to the **Agents window**.

## Steps to reproduce

1. Open the Agents window (`Chat: Open Agents Window` from the Command Palette, or the button in the title bar).
2. Start or open an active agent session.
3. Type `/clear` in the chat input and press Enter.

## Expected behavior

- The current session is marked as **Done** and moves out of the active sessions list.
- A new, empty session opens automatically.
- (This is the behavior that worked previously.)

## Actual behavior

- Nothing happens. The current session remains open and active.
- No new session is created.
- The `/clear` command appears to be silently ignored.

## Additional context

- The same `/clear` command **works correctly** in the regular Chat view (sidebar).
- The issue is isolated to the **Agents window** only.
- The regression appeared approximately in July–August 2026.

## Environment

- **VS Code version:** 1.133.0
- **OS:** Windows
- **Extension:** GitHub Copilot (latest)