Merge TODO.md with git's union merge (closes #98)

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

.gitattributes

@@ -0,0 +1,2 @@
+# Every PR adds an entry at the top of Completed Steps; union keeps both sides.
+TODO.md merge=union