test(e2e): preserve the Launchable platform gateway during cleanup

## Failure evidence

- E2E main retry run `32443766745`, job `96659651958`
- Earlier E2E main run `32439044907`, job `96646144380`
- Image producer run `32443861115`, job `96659978576`, successfully qualified the gateway on a fresh boot before publishing the image

Both NemoClaw jobs booted the producer-qualified image, then failed onboarding because `openshell-gateway.service` was inactive and `gateway.port.uncontested` could not be confirmed.

## Root cause

`test/e2e/live/full-e2e.test.ts` unconditionally destroys the `nemoclaw` gateway during its initial cleanup and registers the same gateway for final cleanup. In `preinstalled-launchable` mode, that gateway is externally supervised by the image platform, so the test stops a resource it does not own immediately before `brev-quickstart` needs it.

## Claim

I am preparing one focused fix that preserves the externally supervised gateway only for the preinstalled Launchable path while retaining sandbox cleanup and the existing source-install cleanup behavior.