Sausage: host, broker, commit-on-close filesystem, cargo actor, console, agent

Imported from the spike branch, with:
- a library crate (Host API) for the CLI and the tests
- JSON test scenarios for candidates
- commit on close only for new or truncated files
- a Nix flake: nix run -- init, nix run -- run

cli/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "sausage"
+version = "0.1.0"
+edition = "2024"
+description = "Grow a system of WASI actors by talking to it."
+
+[[bin]]
+name = "sausage"
+path = "src/main.rs"
+
+[dependencies]
+anyhow = "1"
+bytes = "1"
+clap = { version = "4", features = ["derive"] }
+http = "1"
+http-body-util = "0.1"
+include_dir = "0.7"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tokio = { version = "1", features = ["full"] }
+wasmtime = "49"
+wasmtime-wasi = "49"
+wasmtime-wasi-http = "49"