Agents window: support window.title (or equivalent) for the center title bar text

**Summary**

The Agents window (Preview) title bar intentionally does not render the `window.title` setting. This is explicit in the source code:

https://github.com/microsoft/vscode/blob/main/src/vs/sessions/browser/parts/titlebarPart.ts

```ts
private _windowTitle: WindowTitle | undefined;

get windowTitle(): WindowTitle {
// The Agents window title bar does not render `window.title`, so we
// lazily construct a `WindowTitle` only when a consumer (e.g. a custom
// command center widget) actually asks for one.
if (!this._windowTitle) {
```

By contrast, `workbench.colorCustomizations` tokens for the Agents window shell *are* themeable via `agents.background` / `agentsPanel.foreground` (`src/vs/sessions/common/theme.ts`), so background theming works fine, but the title text does not follow the user's `window.title` template.

**Motivation / use case**

Users who run multiple VS Code profiles/instances for different contexts (e.g. a "private" instance vs. a "work" instance, using separate `--user-data-dir`s) commonly customize `window.title` (e.g. prefixing with an emoji or label like `🔒 PRIVAT — ${activeEditorShort}${separator}${rootName}`) together with `titleBar.activeBackground` colorCustomizations, so it's visually obvious at a glance which instance/profile a window belongs to.

Since the Agents window now supports the background/foreground color tokens (`agents.background`, `agentsPanel.foreground`), it's inconsistent that the same visual-identification mechanism doesn't extend to the title/session-picker text shown in the center `Menus.CommandCenter` toolbar.

**Request**

Please consider either:
1. Honoring `window.title` (or a subset of its variables) for the Agents window's center toolbar text, prefixed/suffixed around the existing session/repo picker, or
2. Introducing a dedicated setting (e.g. `agents.window.title` or similar) that lets users add a custom label/prefix to the Agents window, analogous to how `agents.background` now themes the shell.

This would let users visually distinguish Agents windows across different profiles/instances the same way they already can for regular VS Code windows.

**Environment**

- VS Code version: 1.137.0
- OS: Windows