Pull an NHS Number column out of a CSV file and validate each entry.
The CSV here (`patients.csv`) has three columns: `id`, `name`, and
`nhs_number`. The `run.sh` script strips the header with `tail -n +2`
and picks column 3 with `cut -d, -f3` before piping into the tool.
```sh
./run.sh
```
```sh
For CSVs with quoted fields or embedded commas, use a real CSV parser
(such as `xsv`, `csvkit`, or `miller`) instead of `cut`.