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

[alunduil]

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 Default community health files shared from an alunduil/.github repo 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.