feat: option to keep mappings unlisted or private

A question raised during the lab demo (2026-07-31) was whether mappings could be hidden from other users.
This issue is a record of the question, not a proposal for immediate work.

## Two levels of visibility

### Level 1: Unlisted

Mappings do not appear in **All Mappings**, so other users cannot discover or browse them through the index.

### Level 2: Private

Mappings are stored in a private repository, so only authorized users can access them.

## Current behavior

At the moment, `users.csv` is maintained manually. A user only appears in **All Mappings** after being added to that file, so the current behavior is already effectively opt-in.

However, all `cress-mappings` repositories are public. Not appearing in `users.csv` does **not** make a mapping private: anyone who knows the repository URL can still read the data.

Start with the current users.csv-based inclusion behavior described in the issue. Clarify whether the desired outcome is unlisted discovery control, private repository access, or both; the issue does not define an implementation entry point or acceptance criteria.

Written by the indexing model from the issue text.