Handoff: end a session on a file, not on compaction

Applies to every codebase, and to a plain folder without git. _agent/ at the root is the one folder for everything an agent leaves behind in a repo: working notes in _agent/docs/, these step logs in _agent/handoff/. It is never committed.

Compaction keeps whatever the compactor decides and cannot be inspected. A handoff is a file the human can read, correct and diff, and a feature that spans several sessions becomes an auditable log of steps: step N's Next is step N+1's Done.

The user provided: $ARGUMENTS