chore: add open-source governance files (GPLv3 + contributing docs)

Add standard open-source project files:
- LICENSE.md — GNU GPL v3.0 or later (verbatim)
- CONTRIBUTING.md — setup, workflow, and required AI-usage disclosure
- CONTRIBUTORS.md — maintainers, contributors, AI-assistance policy
- CODE_OF_CONDUCT.md — Contributor Covenant 2.1
- SECURITY.md — private vulnerability reporting
- .gitea/ISSUE_TEMPLATE/* + PULL_REQUEST_TEMPLATE.md
- README: License section (Copyright (C) 2026 Runic Gateway)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XmHdsbnLzDMAVQkAoTQSBe

CONTRIBUTING.md

@@ -0,0 +1,73 @@
+# Contributing to Runic Gateway — Documentation
+
+Thanks for your interest in contributing! This repo is the **central
+documentation** for the Runic Gateway platform — design docs, the protocol spec,
+integration guides, and research, extracted from the code repos so they live in
+one place.
+
+By participating you agree to abide by our
+[Code of Conduct](CODE_OF_CONDUCT.md).
+
+## Ways to contribute
+
+- **Report an error** (something wrong, outdated, or unclear) or **request new
+  documentation** through the
+  [issue tracker](https://gitea.whitlocktech.com/RunicGateway/docs/issues)
+  (issue templates are provided).
+- **Fix or expand the docs** by opening a pull request (see below).
+- **Never** report a security vulnerability in a public issue — see
+  [SECURITY.md](SECURITY.md).
+
+## Working on the docs
+
+Everything here is Markdown — no build step. Just edit the relevant file and
+preview it in any Markdown viewer (or on Gitea).
+
+```
+website/   docs for the shard website (Node/Express + MariaDB + React/Vite)
+link/      docs for the ServUO bridge (C# plugin + Rust sidecar)
+```
+
+Guidelines:
+
+- Keep documents in the folder matching their subsystem (`website/` or `link/`).
+- These docs are the **authoritative** copy of the protocol and design; when you
+  change behavior in a code repo, update the matching doc here in the same or a
+  follow-up PR so the spec and implementation stay in sync.
+- Use relative links between docs, and check that any links you add resolve.
+- Prefer clear prose and tables over screenshots where possible.
+
+## Branch & PR workflow
+
+1. Branch from `main` with a descriptive name (`docs/…`, `fix/…`, `chore/…`).
+2. Keep changes focused; small PRs are easier to review.
+3. Push and open a pull request against `main`. Fill out the PR template,
+   including the **AI-assisted contributions** disclosure.
+4. A maintainer will review; address feedback with follow-up commits.
+
+### Commit messages
+
+We use [Conventional Commits](https://www.conventionalcommits.org/) —
+`type(scope): summary` (e.g. `docs(link): clarify town-crier caps`).
+
+## AI-assisted contributions (disclosure required)
+
+This project is developed openly with AI assistance, and we ask the same
+transparency of everyone. **If you used an AI tool** (Claude, Copilot, ChatGPT,
+Cursor, etc.) to help produce a contribution, you must disclose it:
+
+- Tick the AI-usage box in the pull-request template and name the tool(s).
+- Mark AI-authored commits with a trailer, e.g.
+  `Co-Authored-By: Claude <noreply@anthropic.com>` or `Assisted-By: <tool>`.
+- You remain responsible for every line you submit: review it, understand it,
+  and make sure it is accurate and that you have the right to contribute it.
+
+Disclosed AI assistance is welcome. Undisclosed AI-generated contributions are
+not, and may be closed.
+
+## License
+
+Runic Gateway is licensed under the **GNU General Public License v3.0 or later**
+(see [LICENSE.md](LICENSE.md)). By submitting a contribution you agree that it is
+licensed under the same terms (inbound = outbound) and that you have the right to
+contribute it.