Add Vercel Sandbox as an in-tree provider #144

Proposed integration

Add an in-tree vercel sandbox provider so AgentEnv can deploy environments and agents into Vercel Sandbox. The provider uses an optional Python SDK extra, supports project OIDC or explicit scoped credentials, and implements Docker workloads, port routes, network policy, reconnect and teardown.

The companion PR will include a runnable deterministic evaluation using an existing Vercel CLI login: deploy an items environment, seed its state, run an agent that calls MCP tools, and verify the response, stored state and trajectory.

Please review the resource semantics (CPU/memory requests round up to supported shapes), production image/object stores and network requirements, and maintenance ownership. The evaluation uses disposable image fixtures and local stores; production acceptance remains separate.