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

CHANGELOG.md

@@ -0,0 +1,20 @@
+# Changelog
+
+All notable changes are documented in this file.
+
+## 2026.04.15
+
+### Added
+- Bubblewrap-based Steam launcher with per-user `compatdata` overlay isolation.
+- Activation, uninstall, and add-user helper scripts.
+- Desktop launcher override routing Steam startup through `steam-shared`.
+- Permission repair watcher (`steam-fix-perms.path` + `steam-fix-perms.service`).
+- Nix flake packages for `activate`, `uninstall`, and `add-user`.
+- Basic shell test harness for preflight behavior and release guard checks.
+
+### Changed
+- Activation output now reports the correct desktop file path: `/usr/local/share/applications/steam.desktop`.
+
+### Notes
+- First public product release.
+- Linux-only.