End the current autoresearch run — append a `status:stopped` sentinel to `./.ar/ar.jsonl`, which is also what halts the external `ar-loop.sh` driver. Use for `/ar:stop`, "stop the loop", "halt autoresearch", or "that's enough iterations". Writes the sentinel and the final report, then prints (does not run) the command to return to the original branch. Never reverts or discards work — every kept commit stays on the experiment branch.
