prime-expand

Expand a vague gap into concrete questions + named source candidates via a delegated sub-agent — caller sees only the proposed pair for confirm/edit/refuse, never the expansion reasoning. Trigger /prime-expand, "expand on this", or auto-fire on handoff receipt with a thin brief. (Not for ambient "I need to know X" phrasing — that would collide with /investigate.) Single sub-agent, single turn, ≤3 candidate-source…