Why: the Q3 compiler maps core.WorkflowDef prompt steps onto squad graph
steps; the rendered prompt text needs a first-class carrier in the step
task brief (until now only leader delegations had a briefing channel).
Evidence: TestStepBriefRendersIntoTaskFile asserts the section presence,
body, and position (after targets, before the context table); full squad
suite green (SELF), CI authoritative (GATE).
Impact: additive StepSpec field brief + writeTaskFile section; no behavior
change when Brief is empty.
Refs: Q3 plan WP1 (2026-10-07-agent-stack-q2q3-plans.md); Dev task card
.task/feature-agent-stack-q3.md
