Move Code of Conduct to alunduil/.github as a shared default

## Summary

Create `alunduil/.github` holding the Contributor Covenant 2.1 `CODE_OF_CONDUCT.md`, then delete the per-repo copies so every repo inherits one file.

## Motivation

The 2.1 text is duplicated across repos — collection-json.hs#276 and #114 landed byte-identical copies, and the remaining repos still carry 1.4. Each future revision means one PR per repo. GitHub reads default community health files from a user-level `.github` repo and applies them to any repo that lacks its own copy, so a single file covers all ~29 non-fork repos.

`alunduil/.github` does not exist yet.

## Scope

- Create the public `alunduil/.github` repo with `CODE_OF_CONDUCT.md` at Contributor Covenant 2.1, enforcement contact `alunduil@gmail.com`.
- Delete `CODE_OF_CONDUCT.md` from network-arbitrary and the other repos that carry one. A per-repo file always shadows the default, so the deletions are what makes the shared copy take effect.
- Consider what else belongs there while the repo is being created (`CONTRIBUTING.md`, `SECURITY.md`, issue templates, reusable workflows).

## Acceptance criteria

- [ ] `alunduil/.github` exists and contains the Contributor Covenant 2.1 text.
- [ ] network-arbitrary has no in-tree `CODE_OF_CONDUCT.md` and its community profile resolves the CoC from the default.
- [ ] Remaining repos carrying a per-repo copy are enumerated and cleared.

## Additional context

Follow-up to #114, which upgraded this repo's copy in place. `CODE_OF_CONDUCT.md` is not in `extra-source-files`, so removing it doesn't change the Hackage sdist.

This issue is cross-repo and filed here only because #114 is; it may belong in `alunduil/.github` once that exists.