feat: add SuperServe as a sandbox provider

[mohamedsobhi777]

Mistle already supports several remote sandbox providers (Docker, E2B, Modal,
OpenComputer, Tensorlake).

First-class SuperServe support would let those
teams adopt Mistle without changing their existing sandbox infrastructure.

Proposed solution

Add a superserve provider under packages/sandbox/src/providers/superserve/,
modeled on the existing e2b / tensorlake providers, implementing the three
interfaces in packages/sandbox/src/types.ts:

• SandboxAdapter — start / inspect / resume / stop / destroy /
  captureSnapshot / prepareImage / transparent-proxy config, backed by the
  SuperServe REST API and @superserve/sdk.
• SandboxRuntimeControl — install and drive sandboxd inside the box.
  SuperServe exposes root exec with stdin + env injection (commands.run /
  commands.spawn) and open egress, which is exactly what ensureSandboxd /
  activate need — no black-box constraints.
• SandboxBaseImageBuilder — base image with sandboxd baked in, or
  installed at runtime as the E2B provider does.

We're offering to implement this and open a PR, and SuperServe would maintain
the provider. Since the README notes you aren't accepting contributions yet:
would you be open to an external PR for this?