Write a session handover document in .context/handovers/ so another agent or session can resume the work without re-deriving context. Use when a session is ending (or pausing) with work still in progress, when the user asks to 'write a handover', 'document what's outstanding for the next session', 'hand this off', or 'summarise where we got to', or when a long-running task is about to be interrupted (context limit, user stepping away, switching worktrees). Covers the frontmatter contract, required sections (Session Summary, Completed, Outstanding, Current State, Next Steps, References), the JSON Schema and shell validator that check a handover's shape, and how a follow-up session should pick one up and close it out.