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

.gitea/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,33 @@
+<!--
+  Thanks for contributing to Runic Gateway!
+  Please fill out the sections below and check every box before requesting review.
+-->
+
+## What & why
+
+<!-- What does this PR change, and why? Link any related issue: "Closes #123". -->
+
+## How it was tested
+
+<!-- Commands you ran, manual steps, screenshots. -->
+
+## Checklist
+
+- [ ] I have read [CONTRIBUTING.md](CONTRIBUTING.md).
+- [ ] The change builds and existing tests/checks pass locally.
+- [ ] I have added or updated tests/docs where it makes sense.
+- [ ] My commits are reasonably scoped with clear messages.
+
+## AI-assisted contributions (required)
+
+This project **requires disclosure of AI tool usage**. Please pick one:
+
+- [ ] No AI tools were used to produce this contribution.
+- [ ] AI tools were used. Tool(s): `___________`. I have reviewed and understand
+      every change, and take responsibility for it. AI-authored commits are
+      marked with a `Co-Authored-By` / `Assisted-By` trailer.
+
+## License
+
+- [ ] I agree that my contribution is licensed under this project's license
+      (**GNU GPL v3.0 or later**), and I have the right to contribute it.