treefmt PR check sweeps merge-base drift: main-side unformatted files turn unrelated PRs red #1856

Found 2026-10-28 while triaging code#1759: its treefmt check failed on nixos/home/laptop-repo-keeper.sh and nixos/home/openclaw-backup.sh — files the PR never touches. The check diffs base...head from the PR merge commit, so any unformatted file that lands on main (or drifts into the sweep) makes unrelated PRs red until someone formats main. Two gaps: (1) nothing format-checks main itself on merge, so unformatted files can land (both files arrived via merged PRs whose own checks evidently did not sweep them); (2) the sweep makes the failure surface on the wrong PR. Fix PR for the two current files: fix/treefmt-keeper-scripts. Longer-term options: run the treefmt check on push to main, and/or scope the PR check to files the PR actually changed relative to its merge-base.