[Bug] Agent generates Unix-specific commands (setsid) incompatible with macOS

**Bug Description**
Agent handed me a command to run locally that could not work on this OS (setsid not available on MacOS): ```! sudo setsid nohup sh -c 'while true; do chmod g+w "/Library/Application Support/BOINC Data/slots"/*/shared 2>/dev/null; sleep 5; done' >/dev/null 2>&1 &```

**Environment Info**
- Platform: darwin
- Terminal: iTerm.app
- Version: 2.1.236
- Feedback ID: e7822193-2092-4372-b867-75328313d1e3

**Errors**
```json
[]
```

Start by reproducing the reported command-generation issue on darwin using the provided `setsid` example. Trace where the agent selects platform-specific shell commands; done means commands generated for macOS no longer depend on unavailable Unix utilities, with behavior verified on that platform.

Written by the indexing model from the issue text.