Every PR adds an entry at the top of Completed Steps in TODO.md, so each
merge to next left every other open PR conflicting there. A root
.gitattributes now marks TODO.md merge=union: when two branches insert at
the same place, git keeps the lines of both instead of reporting a
conflict. The cost is that a real conflict elsewhere in TODO.md is no
longer reported either; both versions of the line are kept.
This applies to this repository only; nothing canonical changes.
Model: opus-5-5
