Add Nix flake for local development without Docker/VSCode

Provides flake.nix with nix run, nix develop, and nix build commands.
Automatically sets up lean4game platform and starts game server at
http://localhost:3000. See FLAKE_USAGE.md for complete documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -4,3 +4,6 @@ lake-packages/
 **/.DS_Store
 .i18n/**/*.mo
 *~
+
+# Nix flake build artifacts
+.lean4game-nix/