The packages every harness build is assembled from: the session log, the system-prompt assembly, the tool registry, the agent vocabulary, and the one concrete loop that drives them. These are product packages — the stable surface plugins and consumers build against.

Package	Role	ctx key
`session/`	Event-sourced session log + in-memory store	`ctx.sessions`
`system-prompt/`	Prompt-section + tool-schema assembly registry	`ctx.systemPrompt`
`tools/`	Tool registry + `tools/execute` waterfall	`ctx.tools`
`agent/`	Agent interface, registry, `agent/*` event vocabulary	`ctx.agents`
`agent-loop/`	The concrete loop plugin: `ReactLoopAgent` + the loop driver	`ctx.agentLoop`

agent-loop is the one concrete implementation of the agent seam and lives here because it is the harness's default product loop; everything else in core/ is interface/vocabulary. Plugins depend on the agent vocabulary, never on agent-loop directly, so the loop stays swappable.