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

CONTRIBUTORS.md

@@ -0,0 +1,31 @@
+# Contributors
+
+Runic Gateway is built and maintained by the people and tools listed here.
+Thank you to everyone who has contributed.
+
+## Maintainers
+
+- **whitlocktech** <whitlocktech@gmail.com> — project lead and maintainer
+
+## Contributors
+
+<!--
+  Add yourself here when your contribution is merged — alphabetical by name or
+  handle. One line each:
+
+  - **Name or handle** (optional link) — what you contributed
+-->
+
+- _Your name could be here — see [CONTRIBUTING.md](CONTRIBUTING.md)._
+
+## AI-assisted development
+
+Parts of Runic Gateway were developed with the assistance of AI coding tools,
+including **Claude** (Anthropic) via Claude Code. AI-assisted commits are
+attributed in their commit trailers (e.g. `Co-Authored-By: Claude ...`).
+
+In keeping with this project's transparency policy, **all contributors must
+disclose their use of AI tools** on any contribution — see the
+"AI-assisted contributions" section of [CONTRIBUTING.md](CONTRIBUTING.md).
+Disclosed AI assistance is welcome; undisclosed AI-generated contributions are
+not.