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

TODO.md

@@ -21,6 +21,12 @@ 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 with both entries kept instead of conflicting. The cost
+  is that a real conflict elsewhere in `TODO.md` is no longer reported: both
+  versions of the line are kept. It applies to this repository only; nothing
+  canonical changed.
 - 2026-10-04: The note under the canonical Go `Dockerfile` example in
   `REPO_POLICIES.md` now installs lint-phase system libraries with `apt-get`
   under their Debian package names (issue 83). The `golangci/golangci-lint`