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

examples/notes/tests/basic.json

@@ -0,0 +1,7 @@
+{
+  "description": "add a note, then list the notes",
+  "steps": [
+    {"send": {"type": "add", "text": "test note"}, "expect": {"type": "added"}},
+    {"send": {"type": "list"}, "expect": {"type": "notes"}}
+  ]
+}