strukto-ai/mirage `Workspace.execute` working directory handling

## Summary

`Workspace.execute(command, options)` currently accepts `sessionId`, `agentId`, `signal`, `stdin`, `provision`, `native`, and `noHistory`, but no per-call `cwd` override. To run a command in a different working directory than the session's current `cwd`, callers have to mutate `session.cwd`, run, and restore — which is racy if two `execute()` calls overlap on the same session.

It would be very useful to support a per-call `cwd` directly:

```ts
await ws.execute('ls', { sessionId: 'agent-1', cwd: '/data' });
```

## Why this matters

Agent harnesses and orchestrators routinely need to run a command in a specific directory without permanently changing the session's `cwd`. Today the only safe pattern requires either:

1. Snapshot `session.cwd` → mutate → run → restore in `finally`. Works for serial use but breaks under any concurrency on the same session.
2. Spin up a disposable Mirage session per call, set its `cwd`, and dispose it. Works but is heavyweight, and loses any session-scoped state the caller wanted to share (env, history, function definitions).
3. Shell-prefix with `cd && cmd`. Works but is a workaround, polluting the recorded command history with `cd` noise and forcing every caller to do POSIX-safe path quoting.

A first-class `cwd` option would eliminate all three workarounds and make the API match common expectations from Node's `child_process` and similar libraries.

## Proposed shape

```ts
export interface ExecuteOptions {
// ...exi...