chore: add GitHub community files and CI pipeline

- .github/workflows/ci.yml: TypeScript check + Vitest + build on every PR
- .github/dependabot.yml: weekly npm + Actions updates (minor/patch grouped)
- .github/ISSUE_TEMPLATE/: bug_report, feature_request, config (→ Discussions)
- .github/PULL_REQUEST_TEMPLATE.md: checklist for contributors
- SECURITY.md: responsible disclosure policy via GitHub Advisories
- CONTRIBUTING.md: dev setup, code style, commit conventions

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,27 @@
+## Summary
+
+<!-- What does this PR do? Reference the issue it closes if applicable. -->
+Closes #
+
+## Type of change
+
+- [ ] 🐛 Bug fix
+- [ ] ✨ New feature
+- [ ] 🎨 Style / visual
+- [ ] ♻️ Refactor
+- [ ] 🔒 Security fix
+- [ ] 📝 Documentation
+- [ ] 🛠️ Build / CI
+
+## Checklist
+
+- [ ] `npm run build` passes locally
+- [ ] `npx tsc --noEmit` passes (0 type errors)
+- [ ] `npx vitest run` passes (all tests green)
+- [ ] No secrets / API keys in diff
+- [ ] Documentation updated if needed (`docs/`, `README.md`)
+- [ ] Screenshots attached if there are visual changes
+
+## Screenshots (if applicable)
+
+<!-- Before / After -->