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, so two branches inserting
at the same place merge without a conflict.

Git now never reports a conflict in TODO.md: a real conflict elsewhere
keeps both versions of the line, and two new entries that share an
identical line end up one inside the other. The .gitattributes comment
and the TODO.md entry say to read the merged entries after every merge
or rebase.

This applies to this repository only; nothing canonical changes.

Model: opus-5-5

.gitattributes

@@ -1,3 +1,4 @@
-# Git never reports a conflict in TODO.md: read the merged entries after every
+# Every PR adds an entry at the top of TODO.md's Completed Steps; union keeps
-# merge or rebase, because an entry can land in the middle of another.
+# both sides instead of conflicting. Git never reports a conflict here: read
+# the merged entries after every merge or rebase.
 TODO.md merge=union