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

Cargo.toml

@@ -0,0 +1,5 @@
+[workspace]
+resolver = "3"
+members = ["cli", "agent", "sdk"]
+default-members = ["cli"]
+exclude = ["examples"]