docs: adopt DCO and AI-disclosure contribution policy

CONTRIBUTING.md

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+# Contributing
+
+These are the organization-wide contribution guidelines for
+[agentnameservice](https://github.com/agentnameservice) repositories.
+A repository may extend them with project-specific guidance in its own
+`CONTRIBUTING.md`; where one exists, it takes precedence.
+
+## Pull Requests
+
+- Open an issue first and reference it from your pull request with a closing
+  keyword (e.g. `Fixes #123`) — issues are where triage and prioritization
+  happen.
+- PR titles follow [Conventional Commits](https://www.conventionalcommits.org);
+  release notes are generated from them.
+
+## Developer Certificate of Origin
+
+This organization follows the Linux Foundation contribution model. Every
+commit must carry a `Signed-off-by:` trailer certifying the
+[Developer Certificate of Origin](https://developercertificate.org) — your
+assertion that you have the right to submit the change under the project's
+license. Sign off each commit with:
+
+```
+git commit -s
+```
+
+Missing sign-offs on an existing branch can be fixed with
+`git rebase --signoff origin/main` followed by a force-push. The `DCO`
+status check — the org-wide [DCO app](https://github.com/apps/dco), the
+same enforcement used across Linux Foundation projects — validates this
+on every pull request; bot-authored commits (Dependabot, release-please)
+and merge commits are exempt.
+
+## AI-Assisted Contributions
+
+AI-assisted contributions are welcome **with disclosure**, following the
+[Linux kernel convention](https://docs.kernel.org/process/coding-assistants.html):
+
+- Disclose the tooling in the pull request's **AI assistance** section,
+  naming the tool and model, e.g. `Assisted-by: Claude Code (claude-fable-5)`.
+- AI tools must never add `Signed-off-by:` lines. DCO certification belongs
+  to the human submitter, who remains fully responsible for the correctness
+  and licensing of the contribution.