handoff

Write a restart prompt for the current work to a file, so a fresh session can pick the work up after the context is cleared. Records the goal, what is done and what is next, the decisions already made, the traps found and the uncommitted work, then prints one line to paste after clearing. Use when the user invokes /handoff, or says 'give me a restart prompt so I can clear context', 'write a handoff', 'I want to clear context and continue'.