Run without a clone: bare sausage creates ./world if needed and runs it

- cli: the subcommand is optional; without one, sausage runs ./world and
  runs init first when ./world is not a world yet (test: cli/tests/cli.rs).
- flake: .#loop without a folder runs bare sausage.
- README: nix run git+https://gitea.surma.technology/surma/sausage(#loop),
  clone only for working on Sausage.

flake.nix

@@ -36,6 +36,7 @@
           meta.mainProgram = "sausage";
         };
         # `nix run .#loop -- <world>` runs the newest `sausage run <world>`
+        # (without a folder: bare `sausage`, i.e. ./world, created if needed)
         # from the public repo, again and again: after a crash or a broker
         # `exit` request it refreshes the flake and starts the host again,
         # so the world updates itself. `/quit` (exit code 0) ends the loop.
@@ -44,9 +45,10 @@
         repoUrl = "git+https://gitea.surma.technology/surma/sausage";
         loop = pkgs.writeShellScriptBin "sausage-loop" ''
           url="''${SAUSAGE_FLAKE:-${repoUrl}}"
-          if [ $# -eq 0 ]; then set -- world; fi
+          # No folder: bare `sausage` (creates ./world first if needed).
+          if [ $# -gt 0 ]; then set -- run "$@"; fi
           while true; do
-            nix run --refresh "$url#default" -- run "$@"
+            nix run --refresh "$url#default" -- "$@"
             code=$?
             if [ "$code" -eq 0 ]; then exit 0; fi
             echo "sausage-loop: sausage exited with $code, starting the newest version in 2 s …" >&2