Add prompts README index

Add a short index for the reusable prompts.

## Acceptance criteria

- [ ] Add `prompts/README.md`
- [ ] List each prompt and its intended workflow
- [ ] Include publication-safety reminder
- [ ] Link from `README.md` or `docs/README.md`

## Suggested files

- `prompts/README.md`
- `README.md`
- `docs/README.md`
