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

@@ -0,0 +1,3 @@
+# Git never reports a conflict in TODO.md: read the merged entries after every
+# merge or rebase, because an entry can land in the middle of another.
+TODO.md merge=union

TODO.md

@@ -21,6 +21,14 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-04: Added a root `.gitattributes` that merges `TODO.md` with git's
+  union merge (issue 98), so two branches that each add an entry at the top of
+  Completed Steps merge without a conflict. Git now never reports a conflict in
+  `TODO.md`: a real conflict elsewhere keeps both versions of the line, and when
+  two new entries share an identical line, one is inserted into the middle of
+  the other, which a rebase can do to an entry already on `next`. Read the
+  merged entries after every merge or rebase. This applies to this repository
+  only; no canonical file changed.
 - 2026-10-04: The canonical `.dockerignore` now also keeps out the git `config`
   of a submodule that keeps its own `.git` directory, which still reached the
   image (issue 88): both git patterns now carry the `**/` prefix. A submodule