refactor: launch the VM driver through the generic launcher #5388

Part of #5200 and #5384.

User Story

As an OpenShell Maintainer I want the desktop drivers to exist in their own repository so that their ownership and maintenance can be considered separately to core OpenShell. The VM driver is the first driver to prove the generic launcher.

Problem Statement

The VM driver is launched by VM-specific code in crates/openshell-gateway/src/vm.rs.

Impact / Why This Matters

The VM driver already works as a launched driver, including additional CA trust. Moving it first proves the launcher, startup document, and capability fields before Docker and Podman change.

Proposed Design

The VM driver is launched by the generic launcher with the startup document, replacing the VM-specific spawn and flag-building code. Behavior is unchanged except where the decisions in the launcher issue say otherwise.

Acceptance Criteria

• The VM driver passes its existing e2e, including vm-additional-ca-e2e, with no VM-specific gateway code.
• The trust artifact arrives through the startup document and the driver verifies and snapshots it as before.

Alternatives Considered

• Migrate Docker or Podman first. They have more in-process behavior to replace, so a failure would be harder to attribute to the launcher.