add release artifacts: license, CI, tests, and policy docs

- MIT LICENSE
- CHANGELOG.md (CalVer, first entry 2026.04.15)
- CODE_OF_CONDUCT, CONTRIBUTING, SECURITY, SUPPORT policies
- docs/release.md covering preflight, tagging, rollback
- GitHub Actions CI running shell syntax, shellcheck,
  desktop-file-validate, script tests, and nix flake check
- tests/ harness with activate-path and preflight checks;
  preflight test stubs `id` via PATH so it cannot launch real
  Steam on a developer machine where /opt/steam already exists

CODE_OF_CONDUCT.md

@@ -0,0 +1,15 @@
+# Code of Conduct
+
+## Our Standard
+
+Contributors keep communication respectful, direct, and constructive.
+Harassment, discrimination, and hostile behavior are not accepted.
+
+## Enforcement
+
+Project maintainers review reports and take action based on impact and severity.
+Actions can include warnings, removal of content, temporary bans, or permanent bans.
+
+## Reporting
+
+Open a private security advisory or contact maintainers through GitHub with the relevant context.