Write a .bidsignore for the campaign

The campaign carries no `.bidsignore` today. Per `docs/output_structure.md` it should hold:

```
studies/
derivative-attempts/
```

`studies/` is ignored because BIDS describes no study *containing* studies — a spec gap rather than a mechababs choice.

Start by reading docs/output_structure.md and inspect the campaign’s existing top-level files to confirm where .bidsignore belongs. Add the two entries shown in the issue, then verify that the campaign contains the file with exactly those ignored paths.

Written by the indexing model from the issue text.