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.