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

README.md

@@ -38,3 +38,21 @@ link/      docs from the ServUO bridge (C# plugin + Rust sidecar + Node WS)
 Commit history and authorship for each doc are preserved. The two source repos
 retain a short pointer to this repo in their own READMEs; the authoritative copy
 of each document now lives here.
+
+---
+
+## License
+
+Runic Gateway's documentation is free: licensed under the **GNU General Public
+License v3.0 or later** — see [LICENSE.md](LICENSE.md).
+
+    Copyright (C) 2026 Runic Gateway
+
+    This documentation is distributed in the hope that it will be useful, but
+    WITHOUT ANY WARRANTY. You may redistribute and/or modify it under the terms of
+    the GNU General Public License as published by the Free Software Foundation,
+    either version 3 of the License, or (at your option) any later version.
+
+Contributions are welcome — please read [CONTRIBUTING.md](CONTRIBUTING.md) (note
+the **AI-usage disclosure** requirement) and our
+[Code of Conduct](CODE_OF_CONDUCT.md).