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

Every PR here adds an entry at the top of Completed Steps in `TODO.md`, so each merge to `next` left the other open PRs conflicting there. A root `.gitattributes` now marks `TODO.md` with `merge=union`: when two branches insert at the same place, git keeps both sides. It applies to this repository only; nothing canonical changes.

Union never reports a conflict in `TODO.md`. When two new entries share an identical line, one can land inside the other, and a rebased entry sits below every entry that reached `next` after the branch was cut, so the merged entries are read after every merge or rebase. Whether Gitea's own conflict check applies the attribute is not known.

Model: opus-5-5

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