stdin content doesn't get sent

- `matrix-commander -m "First message!"` works so I'm definitely logged in.
- `df -h | matrix-commander --code` sends nothing.
- `echo 123 | matrix-commander` sends nothing.

```
matrix-commander --version

_| _| _|_|_| _| matrix-commander: 8.0.6 2026-07-02
_|_| _|_| _| _| a Matrix CLI client
_| _| _| _| _| enjoy and submit PRs
_| _| _| _| matrix-nio: 0.25.2
_| _| _|_|_| _| Python: 3.14.6
```

Installed via pip.

Look at the stdin handling code in the main entry point, likely in matrix_commander.py or a similar file. Check how the program reads from stdin when piped input is provided. Run the failing commands locally to reproduce the issue, then trace through the logic to see where the stdin content is being dropped or not processed.

Vom Indexierungsmodell aus dem Issue-Text verfasst.