Allow configuring the default Changes view changeset

The Agents Changes view currently selects a provider-defined default changeset (for example, Branch Changes or Session Changes). A user can manually select Last Turn Changes, but that selection is reset when the active session changes.

Please add a user setting that controls the default changeset shown in the Agents Changes view.

For example:

"agents.changes.defaultChangeset": "lastTurn"

Possible values could include:

• branch
• uncommitted
• session
• lastTurn

The setting should be honored whenever the Changes view initializes or switches to another agent session. If the selected changeset is unavailable for a particular provider/session, VS Code could fall back to that provider's current default.

My use case is reviewing an agent incrementally after every prompt. Showing the cumulative branch or session diff by default makes it harder to identify exactly what the latest turn changed, and repeatedly switching the selector to Last Turn Changes adds friction.

An alternative would be to remember the user's most recently selected changeset mode across sessions/workspaces, but an explicit setting would be more predictable and configurable through Settings Sync.

Start by locating the Agents Changes view and the code that initializes its changeset or responds to agent-session switches. Add the user setting with the proposed changeset values, honor it during initialization and session changes, and preserve the provider default when the selected changeset is unavailable.