[py] Add CLI entrypoint to chat with a data source

It could be neat to be able to quickly spin up a querychat instance against a `.csv`  or `.parquet` file, or a database, e.g.

```bash
uvx querychat titanic.csv
uvx querychat sales.parquet

uvx querychat --client claude/claude-haiku-4-5-20251001 titanic.csv
```