strukto-ai/mirage `Workspace.execute` environment variable handling

## Summary

`Workspace.execute(command, options)` currently has no way to inject per-call environment variables. The two available paths both have problems:

1. **Mutate `session.env` then restore** — racy if two `execute()` calls overlap on the same session, and forces every caller to write their own snapshot/restore boilerplate.
2. **Use a shell command-prefix like `FOO=bar cmd`** — silently produces wrong results today (see "Workaround doesn't work" below), so this isn't a real workaround.

A first-class `env` option would eliminate both problems:

```ts
await ws.execute('printenv FOO', { sessionId: 'agent-1', env: { FOO: 'bar' } });
```

## Why this matters

Agent harnesses pass per-call env all the time — feature flags, scoped credentials, request-scoped tokens. Without a first-class option, callers either own the snapshot/restore complexity themselves, or are forced into the third option below, which doesn't actually work.

## Workaround doesn't work — context for why this is important

In other shells, `FOO=bar cmd` is the standard fallback when you can't pass env via API. On Mirage today, this is silently broken:

```ts
await ws.execute("FOO=bar printenv FOO");
// exit: 1, stdout: "", stderr: ""
// FOO=bar was passed as argv to printenv, not as env

await ws.execute("FOO=bar echo \$FOO");
// exit: 0, stdout: "echo\n"
// $FOO expanded to empty before echo ran; argv became ['FOO=bar', '']
// so echo printed FOO=bar... actually no, it printed "echo" — because
// classify...