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

CONTRIBUTING.md

@@ -0,0 +1,24 @@
+# Contributing
+
+## Development Workflow
+
+1. Create a branch from `main`.
+2. Implement a focused change.
+3. Run local verification.
+4. Open a pull request.
+
+## Required Local Checks
+
+```bash
+shellcheck scripts/*.sh tests/*.sh
+bash -n scripts/*.sh tests/*.sh
+desktop-file-validate desktop/steam.desktop
+tests/run.sh
+XDG_CACHE_HOME=/tmp nix flake check --no-build --no-write-lock-file
+```
+
+## Scope Rules
+
+- Keep changes minimal and focused.
+- Add or update tests for behavior changes.
+- Do not commit secrets.