Code of Conduct comes from the shared alunduil/.github defaults

## Summary

`CODE_OF_CONDUCT.md` is gone from this repo, and the Code of Conduct GitHub shows for siren-json.hs resolves from the shared defaults in `alunduil/.github`, so the text is maintained in one place across repos.

## Motivation

A per-repo copy makes every Contributor Covenant revision an N-repo change: #79 upgrades siren-json to 2.1 and collection-json.hs tracks the identical upgrade as its #151. GitHub resolves community health files from a `.github` repository owned by the account whenever the target repo has no file of that type, so one file there covers every repo.

## Prerequisites

`alunduil/.github` does not exist yet (404 at filing time). Before this can be worked:

- The repo exists and is **public** — GitHub ignores defaults from a private `.github` repository.
- It carries `CODE_OF_CONDUCT.md` in the root, `.github/`, or `docs/`, at Contributor Covenant 2.1 with a reachable enforcement contact.

## Scope

- Delete `CODE_OF_CONDUCT.md`.
- Repoint `CONTRIBUTING.md:8`. It links `CODE_OF_CONDUCT.md` as a relative path, which 404s once the file is gone, so the shared copy needs an absolute URL.

## Acceptance criteria

- [ ] `CODE_OF_CONDUCT.md` is absent from the repo.
- [ ] shows a Code of Conduct sourced from `alunduil/.github`.
- [ ] No link in the repo resolves to a missing `CODE_OF_CONDUCT.md`.

## Additional context

#79 upgrades the local file to 2.1 and corrects the enforcement contact. That work isn't wasted — it settles what text and address `alunduil/.github` should carry — but the deletion here is independent of it.

`CONTRIBUTING.md` stays repo-local: it carries siren-json's cabal build, test, and publish steps, which aren't shareable.

`siren-json.cabal`'s `extra-doc-files` doesn't list `CODE_OF_CONDUCT.md`, so the sdist tarball CI builds from is unaffected by the deletion.

First check whether the public alunduil/.github repository exists and provides the required Code of Conduct; the issue says it did not exist when filed. Inspect CODE_OF_CONDUCT.md, CONTRIBUTING.md at line 8, and repository links, then remove the local file and update the link. Done means the shared GitHub community page resolves, no repository link is broken, and the local file is absent.

Rédigé par le modèle d'indexation à partir du texte de l'issue.