The session log, system-prompt assembly, tool registry, agent vocabulary, and concrete loop that form the harness's default control spine. These are **product** packages — the stable surface plugins and consumers build against.
`scope` supplies the shared scoping primitive. `agent` owns the public seam, while `agent-loop` is its default implementation; extension plugins depend on the seam so the driver remains swappable.
Runnable compositions belong to [`examples/agent-spine-demo`](../examples/agent-spine-demo/README.md); this group owns only the swappable spine pieces.