Add a code of conduct

## Why This project is starting to invite outside contributors, including people who will be asked to make review judgements about research artifacts. It should state plainly how participants are expected to treat each other, and what happens if that breaks down. There is currently no `CODE_OF_CONDUCT.md`. ## The task Add a code of conduct. The [Contributor Covenant](https://www.contributor-covenant.org/) v2.1 is the conventional choice and is what most contributors will expect to see. Two things need a real decision rather than a copy-paste: 1. **Enforcement contact.** The template has a placeholder. It needs a working address that the maintainer is willing to publish. 2. **Research-specific conduct.** This project asks people to disagree with each other's evidence. Consider adding a short section stating that criticism of a claim, a method, or a result is welcome and expected, while criticism of a person is not — and that disclosing a conflict of interest or prior exposure is treated as good practice rather than as an admission of fault. The second point matters here more than in a typical repository, because `CONTRIBUTING.md` already requires reviewers to disclose prior exposure. People need to know that disclosing will not be held against them. ## Acceptance criteria - [ ] `CODE_OF_CONDUCT.md` exists at the repository root. - [ ] The enforcement contact is filled in, not a placeholder. - [ ] `CONTRIBUTING.md` links to it. - [ ] The research-criticism section is present, or this issue records a reasoned decision to omit it. ## Getting started No code and no research background needed. A good first contribution if you care about how a community is set up.