Broker exit request and nix run .#loop for self-updating hosts

- broker: `{"type":"exit"}` answers `exiting`; `sausage run` then stops all
  actors and exits with code 75.
- flake: apps `loop` / `loop-internal` run `nix run --refresh git+ssh://.../surma/sausage -- run <world>`
  in a loop; exit code 0 (/quit) ends the loop, anything else restarts.
- PROTOCOL.md, README.md, test in cli/tests/host.rs.

Not compiled: written by the Sausage agent, which cannot build the native host.

README.md

@@ -23,6 +23,16 @@ the base URL, the model, and the token. It reads `SAUSAGE_API`,
 values go to `world/actor/agent/data/config.json`. Both commands take another
 folder as an argument, for example `nix run -- run my-world`.
 
+To keep a world on the newest version, run it in a loop:
+
+```sh
+nix run .#loop -- my-world            # or .#loop-internal (Gitea on the LAN)
+```
+
+The loop runs `nix run --refresh git+ssh://…/surma/sausage -- run my-world`.
+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.
+
 In the console:
 
 ```text