meta: CODEOWNERS, PR template, issue forms, SECURITY.md, CONTRIBUTING.md

[benjamineckstein]

Summary

• CODEOWNERS — routes every PR to @benjamineckstein for auto-review request
• PR template — checklist: tests, Core purity, Godot compile, headless balance check
• Issue templates — structured YAML forms for bug reports and feature requests
• SECURITY.md — private disclosure via email, 72h response commitment
• CONTRIBUTING.md — prerequisites, run instructions, contribution flow, hard rules

Test plan

• Docs-only change — dotnet test still passes (1025 / 1025 ✅ verified locally)
• Issue forms render correctly on GitHub (YAML form format)
• PR template appears when opening a new pull request