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

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 Felix Foertsch
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.