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.

README.md

@@ -11,31 +11,43 @@ change its own code.
 ## Start
 
 You need [Nix](https://nixos.org/download) with flakes, and an API token for
-an LLM (OpenAI Responses or Anthropic Messages API).
+an LLM (OpenAI Responses or Anthropic Messages API). No clone needed:
 
 ```sh
-git clone https://gitea.surma.technology/surma/sausage && cd sausage
-nix run -- init     # creates ./world and builds the agent (a few minutes)
-nix run -- run      # starts the world and a console
+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) and then starts it with a console. Later runs start the
+existing world. The two steps also exist on their own:
+
+```sh
+nix run git+https://gitea.surma.technology/surma/sausage -- init   # create ./world
+nix run git+https://gitea.surma.technology/surma/sausage -- run    # start it
 ```
 
 `init` asks for the API shape (`openai-responses` or `anthropic-messages`),
 the base URL, the model, and the token. It reads `SAUSAGE_API`,
 `SAUSAGE_BASE_URL`, `SAUSAGE_MODEL`, and `SAUSAGE_API_TOKEN` first. The
-values go to `world/actor/agent/data/config.json`. Both commands take another
-folder as an argument, for example `nix run -- run my-world`.
+values go to `world/actor/agent/data/config.json`. `init` and `run` take
+another 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 .#loop -- my-world
+nix run git+https://gitea.surma.technology/surma/sausage#loop             # ./world
+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`.
+The loop runs the newest `sausage` from that URL (`nix run --refresh`).
 When the host crashes or an actor sends `@broker {"type":"exit"}`, the loop
 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.
 
+To work on Sausage itself, clone it
+(`git clone https://gitea.surma.technology/surma/sausage`) and use `nix run`,
+`nix run .#loop` and `nix develop` in the checkout.
+
 In the console:
 
 ```text