Code of Conduct is inherited from alunduil/.github instead of carried locally

## Summary

Delete this repo's `CODE_OF_CONDUCT.md` so it falls through to the shared default in `alunduil/.github`, and rewrite `CONTRIBUTING.md`'s link to it as an absolute URL.

## Motivation

alunduil/alunduil-infrastructure#213 stands up `alunduil/.github` as the single home for community-health boilerplate. Once that lands, this repo's copy is redundant: #276 replaced it with the generic Contributor Covenant 2.1 carrying the `alunduil@gmail.com` contact, which is exactly the baseline text #213 plans to default. Keeping a local copy means it silently wins over the shared one and drifts again.

## Scope

- Delete `CODE_OF_CONDUCT.md`.
- Rewrite `CONTRIBUTING.md:7`'s `[Code of Conduct](CODE_OF_CONDUCT.md)` to an absolute URL. Default community health files never appear in the repo's file browser, git history, clones, or downloads, so a relative link 404s the moment the local file goes.

## Acceptance criteria

- [ ] `CODE_OF_CONDUCT.md` no longer exists in this repo.
- [ ] This repo's Community Standards page shows the code of conduct inherited from `alunduil/.github`.
- [ ] `CONTRIBUTING.md`'s code-of-conduct link resolves; lychee passes in `links.yml`.

## Additional context

- Blocked by alunduil/alunduil-infrastructure#213 — nothing to do here until `alunduil/.github` is live and Terraform has applied.
- No Hackage impact. `collection-json.cabal`'s `extra-doc-files` ships `CHANGELOG.md`, `COPYRIGHT`, and `README.md`; `extra-source-files` ships `Setup.hs`. The code of conduct was never in the sdist.

Wait for alunduil/alunduil-infrastructure#213 and confirm the shared community-health files are live. Then delete CODE_OF_CONDUCT.md, update the link on CONTRIBUTING.md:7 to the specified absolute URL, and run the lychee check configured in links.yml. Done means the Community Standards page inherits the shared conduct policy and the link check passes.

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