the built-in archetypes come with a default prompt

docs/2-handbook/swarm/archetypes.md

@@ -81,6 +81,10 @@ archetype without `prompt` leaves the box alone.
   `refused` map. A blank `prompt` counts as none.
 - The text is a copy made at create. Editing the archetype's `prompt` later
   changes what new agents start with, not the `AGENTS.md` of agents that exist.
+- Each built-in archetype comes with a default prompt. It describes who the
+  agent is and what it cares about, and names no repo, agent or swarm path.
+  Where the files live:
+  [the implementation page](../../4-implementation/swarm/archetypes.md#the-built-in-prompts).
 
 ## How the list stays current