The session log, system-prompt assembly, tool registry, agent vocabulary, deployment-default model selection, and concrete loop that form the harness's default control spine. These are product packages — the stable surface plugins and consumers build against.

Package	Role	ctx key
`scope/`	Scoped-context registration primitive	library — no ctx key
`session/`	Event-sourced session log and in-memory store	`ctx.sessions`
`system-prompt/`	Prompt and tool-schema assembly registry	`ctx.systemPrompt`
`tools/`	Scoped tool registry and execution pipeline	`ctx.tools`
`agent/`	Agent interface, registry, and event vocabulary	`ctx.agents`
`agent-default-model/`	Default model selection shared by Agent entry points	`ctx.agentDefaultModel`
`agent-loop/`	Default concrete agent driver	`ctx.agentLoop`

scope supplies the shared scoping primitive. agent owns the public contract, while agent-loop is its default implementation; extension plugins depend on the seam so the driver remains swappable. agent-default-model owns the deployment selection an Agent entry point uses only when a session has no selection of its own.

Runnable compositions belong to examples/agent-spine-demo; this group owns only the swappable spine pieces.

The subsystem reference — the package-by-package loop map, the Agent handle and its delivery/interception contracts — is docs/subsystems/core.md; the default runnable composition is examples/agent-spine-demo.