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/feature_request.md

@@ -0,0 +1,23 @@
+---
+name: Feature request
+about: Suggest an idea, enhancement, or new capability
+title: "[feature] "
+labels:
+  - enhancement
+---
+
+## Problem / motivation
+
+<!-- What are you trying to do? What's missing or painful today? -->
+
+## Proposed solution
+
+<!-- What you'd like to see happen. -->
+
+## Alternatives considered
+
+<!-- Other approaches you thought about, and why you prefer the one above. -->
+
+## Additional context
+
+<!-- Mockups, links, related issues, affected component/repo, etc. -->