Let the agent/SDK detach (promote to top-level) and pin sessions programmatically

### Feature summary

Give the agent (and the app SDK) a way to promote a session to top-level and pin it programmatically — the same "detach" + "pin" actions that exist in the UI today.

### What problem are you trying to solve?

I orchestrate work across several dedicated, long-lived sessions (control dashboards, a repo-owner session, etc.). When the agent creates one of these via `create_session`, it always nests under the creating session. To make these dedicated sessions easy to find and durable, I want them to be **top-level and pinned**.

Right now that's only possible manually through the UI ("detach" then "pin" on each session). There is no agent tool or SDK call to do it, so any automated bootstrap flow (create the dedicated sessions on a new machine) leaves them buried as nested children until I fix the hierarchy by hand.

### Proposed solution

Expose the existing UI capabilities to the agent/SDK, e.g.:

- A tool/SDK method to **detach** a session (reparent to top-level), and
- A tool/SDK method to **pin/unpin** a session.

Ideally `create_session` could also accept optional flags like `top_level: true` and `pinned: true` so a dedicated session can be created in its final placement in one step.

### Workflow impact

Makes agent-driven, multi-session topologies fully self-setting-up: the bootstrap step can create the dedicated sessions AND place/pin them correctly, so a new machine ends up with the same clean, pinned top-level layout without any manual UI fiddling.

### Installation context

GitHub Copilot desktop app on Windows. Multi-session orchestration created via the agent's `create_session` tool.

### Additional context

Related to a canvas-bootstrap request (project canvases in the "Start from canvas" picker) — together they'd let a project ship a portable, self-bootstrapping, self-organizing multi-session control setup.

Beginne damit, das create_session-Tool des Agenten und die vorhandenen UI-Aktionen zum Ablösen und Anheften von Sitzungen nachzuverfolgen. Definiere die Agent-/SDK-Einstiegspunkte und das Sitzungsverhalten, die erforderlich sind, damit neu erstellte Sitzungen auf oberster Ebene und angeheftet werden können, einschließlich der vorgeschlagenen create_session-Flags; abgeschlossen ist die Aufgabe, wenn die Platzierung ohne manuelle UI-Schritte festgelegt werden kann.

Vom Indexierungsmodell aus dem Issue-Text verfasst.