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/ISSUE_TEMPLATE/config.yaml

@@ -0,0 +1,5 @@
+blank_issues_enabled: true
+contact_links:
+  - name: Security vulnerability
+    url: https://gitea.whitlocktech.com/RunicGateway/website/src/branch/main/SECURITY.md
+    about: Please do not open a public issue for security problems — report them privately by email instead (see SECURITY.md).