`jj | head` exits with non-zero code since `head` breaks the
pipe. Also, removed `--color=always` from that command as it
will shortly become unnecessary.
Previosly, this caused the script to stop since it's run with
`set -o pipefail`.
Also, the operation id recovery code stopped working. We
can use `jj debug operation` for this purpose now.
I think it's clearer if only the actual demos started with `demo_`,
so I renamed `demo_helpers.sh` to just `helpers.sh`.
`demo_resolve_conflict.sh` should match `resolve_conflicts.png` (with an s).
