handoff-execute

Execute a handoff brief with enforced source-read discipline — load the brief's @SOURCES.required tier fully before planning, cite sources per plan step, run @OUTCOME.verify after, emit the retro per @GROUND_TRUTH_REQUIRED. Trigger when the user runs /handoff-execute <brief-path>, or pastes the activation phrase emitted by /handoff. Not auto-triggered — the receiving session invokes explicitly. Use /plan-execute for…