feat: add opsen as a sandbox provider

[md322613]

Adds @vibe-kit/opsen, following the structure of packages/e2b.

opsen runs agent sandboxes and proxies their model calls, so machine, model and tool spend land under one task id.

const vibeKit = new VibeKit()
  .withAgent({ type: "grok", provider: "xai", apiKey: process.env.XAI_API_KEY! })
  .withSandbox(createOpsenProvider({
    apiKey: process.env.OPSEN_API_KEY!,
    taskId: "code-review",
    budgetUsd: 2.0,
  }));

Why another provider. Every sandbox in the current list bills for the machine and leaves the model bill with a different vendor — VibeKit's agent config takes a separate model provider and API key, so a team running Grok on a sandbox holds two bills with no shared identifier. Here instance.cost() returns machine, model and tool spend under one task id, and budgetUsd is a ceiling enforced mid-run rather than a number found on an invoice.

Agent attribution. VibeKit runs grok, claude, codex, gemini and opencode through one provider, so agentType is written as a label at session creation. "Which agent cost what" is only answerable if it is recorded when the session starts.

Interface. Implements SandboxProvider and SandboxInstance, with the interfaces declared locally as the other providers do. Typechecks clean against the published @opsendev/opsen client.

One limitation, stated plainly. getHost(port) depends on the runtime underneath. It works on E2B, works on Modal when the port was declared at sandbox creation, and throws with a message naming the limitation where unavailable, rather than returning a URL that does not answer.

Also adds docs/supported-sandboxes/opsen.mdx and its docs.json nav entry, following e2b.mdx.

Happy to restructure to match house conventions.

[homanp]

@md322613 thanks for the PR, will have a look

[md322613]

Anything I can do to move this along? Happy to rebase, split it up, or adjust to conventions I have missed.

If workspace sandbox providers are not something you are taking right now, that is a fine answer and I would rather know than leave it open.