Instead of squash-merging into main, `sandlot squash` now soft-resets
to the merge base and recommits, keeping the branch independent.
This removes the --force flag and all squash-merge logic from
mergeAndClose, which goes back to being a plain merge.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Change the commit message generation prompts from requesting
50/72-rule multi-line messages to single-line subjects only.
This avoids unnecessary body text in automated commits and
ensures the first line is always used via `.split("\n")[0]`.
