chore: unused import os in cli.py (ruff F401)

## Summary

`import os` on line 15 of `cli.py` is never used. `ruff check` reports it as F401.

```
src/harnessbench/cli.py:15:8 F401 `os` imported but unused
```

## Fix

Remove line 15:

```python
# delete this line
import os
```

Fixable automatically with:

```bash
ruff check --fix src/harnessbench/cli.py
```

## Affected file

`src/harnessbench/cli.py`, line 15