feat(sdk): add sandboxId-first top-level sandbox operations

[nicoalbanese]

Open Harness hacking with @LukeSheard.

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Summary

Expose sandbox operations as top-level SDK APIs so callers can work directly with a sandboxId without first creating a Sandbox instance.

What changed

• added a dedicated src/sandbox-operations.ts module for standalone sandbox helpers
• updated standalone helpers to accept sandboxId as the first argument
• added optional { client } as the last argument so callers can reuse an existing client
• updated Sandbox instance methods to delegate to the standalone helpers while passing the instance client
• re-exported the standalone helpers from the package entrypoint

Example

await mkdir(sandboxId, "tmp", { client });

await writeFile(
  sandboxId,
  { path: "tmp/hello.txt", content: Buffer.from("hello") },
  { client },
);

const cmd = await runCommand(
  sandboxId,
  "cat",
  ["tmp/hello.txt"],
  { client },
);

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