Opt out flow #23

Why

Anyone should be able to remove their project or handle from the index.

What to build

• An /opt-out page (pages/opt-out.tsx, registered in App.tsx) with the project id (prefilled from a link on the project page) and a GitHub handle. Submitting files an issue on this repo through the API with a fixed template, using a dedicated token setting that can create issues, and shows "we honor opt outs within 24h".
• A hidden list in the hackathon YAML, with the field added to the Hackathon model in hackathon.py. A maintainer applies an opt out by adding the project id.
• Hidden projects, and the builder's handle, are skipped everywhere: the API (queries.py), the export (export/build.py) and the chat tools.

Acceptance criteria

• A hidden id never appears in any API response, the JSON and YAML export, or chat tool results, with a test for each.

See CONTRIBUTING.md for conventions.