Drop the TODO.md union-merge rule (closes #169)

The rule and its comment existed only so concurrent PRs could each add a
Completed Steps entry to `TODO.md`. That work log is gone; `TODO.md` now
holds only open work, so it merges normally again.

Model: opus-5-5

.gitattributes

@@ -1,7 +1,2 @@
-# Every PR adds an entry at the top of TODO.md's Completed Steps; union keeps
-# both sides instead of conflicting. Git never reports a conflict here: read
-# the merged entries after every merge or rebase.
-TODO.md merge=union
-
 # yarn's release file is one opaque file in diffs and merges.
 .yarn/releases/** binary