Agents window should support the same local Copilot agent harness as the Chat view

The Agents window should support the same local Copilot agent harness that is available in the normal VS Code Chat view/sidebar.

Today, the experience appears split:

* The Chat view/sidebar has the native local Copilot Agent experience, with workbench context, VS Code tools, extension-provided tools, MCP/model configuration, and integrated terminal behavior.
* The Agents window is documented as the agent-focused surface, but it appears centered on Copilot CLI, Copilot Cloud, and other agent-host style sessions instead of the same local Copilot Agent harness used by the Chat view.

Expected:

* The Agents window can start and run the same local Copilot Agent harness as the Chat view/sidebar.
* Users do not have to switch to a different backend, such as Copilot CLI, just to use the agent-focused window.
* Local Copilot Agent sessions and Agents window sessions can share history/continuation where compatible.
* If the Agents window intentionally uses a different backend for now, the UI and docs should make that clear.

Why this matters:

The Agents window is a good fit for managing parallel agent work across projects, but using it should not mean giving up the first-class local Copilot Agent experience from the Chat view. As a user, I expect these to be two views over the same core Copilot agent experience, not separate products with different capabilities and histories.

Related issues:

* #325132 tracks local Copilot Chat Agent sessions not being visible/continuable in the Agents window.
* #317895 tracks MCP configuration and BYOK model parity gaps for the Agents window local agent.
* #319415 tracks Copilot CLI sessions appearing in the Chat Sessions panel but not in the Agents window.
* #323969 tracks session-reference attachment support across chat/session types.
* #323970 tracks full session porting for Continue in Agent Host, including local/workbench chat sessions.

Start by comparing the local Copilot Agent experience in the Chat view/sidebar with the Agents window, then read related issues #325132, #317895, #319415, #323969, and #323970. The scope is complete when the Agents window can use the local harness with compatible history and continuation, or when its different backend and limitations are clearly documented.

Written by the indexing model from the issue text.