Bare sausage creates ./world if needed and runs it (default app)

- 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).
  So `nix run git+https://gitea.surma.technology/surma/sausage` works.
- flake: .#loop without a folder runs bare sausage.
- README: mention the bare command and the bare loop.

README.md

@@ -14,7 +14,15 @@ You need [Nix](https://nixos.org/download) with flakes, and an API token for
 an LLM (OpenAI Responses or Anthropic Messages API). No clone needed:
 
 ```sh
-nix run git+https://gitea.surma.technology/surma/sausage -- init   # creates ./world and builds the agent (a few minutes)
+nix run git+https://gitea.surma.technology/surma/sausage
+```
+
+This creates a world in `./world` the first time (it builds the agent, which
+takes a few minutes), then starts it with a console. Later runs start the
+existing world. The two steps on their own:
+
+```sh
+nix run git+https://gitea.surma.technology/surma/sausage -- init   # creates ./world and builds the agent
 nix run git+https://gitea.surma.technology/surma/sausage -- run    # starts the world and a console
 ```
 
@@ -27,7 +35,8 @@ folder as an argument, for example `... -- run my-world`.
 To keep a world on the newest version, run it in a loop:
 
 ```sh
-nix run git+https://gitea.surma.technology/surma/sausage#loop -- my-world
+nix run git+https://gitea.surma.technology/surma/sausage#loop               # ./world, created if needed
+nix run git+https://gitea.surma.technology/surma/sausage#loop -- my-world   # an existing world
 ```
 
 The loop runs `nix run --refresh git+https://gitea.surma.technology/surma/sausage -- run my-world`.
@@ -36,7 +45,8 @@ fetches the newest commit and starts it again. `/quit` ends the loop. Set
 `SAUSAGE_FLAKE` to run another flake URL instead, for example your fork.
 
 From a clone (`git clone https://gitea.surma.technology/surma/sausage`), the
-same commands are `nix run -- init`, `nix run -- run` and `nix run .#loop`.
+same commands are `nix run`, `nix run -- init`, `nix run -- run` and
+`nix run .#loop`.
 
 In the console: