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.

cli/src/main.rs

@@ -92,9 +92,20 @@ async fn init(dir: PathBuf) -> Result<()> {
 async fn run(dir: PathBuf) -> Result<()> {
     let host = Host::start(&dir).await?;
     tokio::spawn(console::run_mailbox(host.mailbox(console::ID)));
-    console::run_input(host.broker.clone()).await;
+    let broker = host.broker.clone();
+    let exit = tokio::select! {
+        _ = console::run_input(host.broker.clone()) => false,
+        _ = broker.exit_requested() => true,
+    };
     println!("Stopping actors …");
     host.stop().await;
+    if exit {
+        // The console still blocks on a stdin read that cannot be cancelled,
+        // so the runtime would not shut down on its own. Exit code 75 tells
+        // `nix run .#loop` to start the host again (0 = /quit = stop the loop).
+        println!("Exiting, as an actor asked the broker to.");
+        std::process::exit(75);
+    }
     Ok(())
 }