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

SUPPORT.md

@@ -0,0 +1,16 @@
+# Support
+
+## Where to Ask for Help
+
+Use GitHub Issues for installation help, troubleshooting, and feature requests.
+
+Include:
+- distribution and kernel version
+- native Steam or Flatpak Steam
+- command output and exact error text
+- steps already tried
+
+## Scope
+
+This project supports Linux shared-library usage with `/opt/steam` and a `steamshare` group.
+Other operating systems are out of scope.