Record/refine a live plan (to-do checklist) for the current Turn (epic D.1/D.3).
This is the canonical plan emit site: it publishes an ephemeral :plan Event on
the session bus, which the ACP front-end renders as a live checklist. It mutates
no files and runs no commands, so it is permitted even in :read_only/plan mode
(the architect tool). The model calls it in plan mode to present its plan, then
stops (plan-and-wait).