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-v2/tests/basic.json

@@ -0,0 +1,9 @@
+{
+  "description": "version 2 counts notes and no longer crashes",
+  "steps": [
+    {"send": {"type": "add", "text": "test note"}, "expect": {"type": "added"}},
+    {"send": {"type": "list"}, "expect": {"type": "notes"}},
+    {"send": {"type": "count"}, "expect": {"type": "count", "version": 2}},
+    {"send": {"type": "crash"}, "expect": {"type": "not_crashing"}}
+  ]
+}