Handoff artifact

One small file capturing where the work stands, so a fresh session resumes with no re-explanation. Written at roughly half the window rather than when it fills, because the oldest details fall away and long-chain reasoning degrades before the hard limit is ever reached. Two rules keep it useful: overwrite rather than append, since an appended handoff decays into a transcript nobody reads; and stay a pointer rather than a transcript, linking the durable records instead of copying them.