docs: a change adds no Completed Steps entry to TODO.md (closes #222)

Every change used to add an entry at the top of Completed Steps in its
own commit, so each merge into `next` left every other open PR
conflicting on that list. The Workflow section now says to add no
entry: what landed is recorded on the tracker and in `git log`, where
each commit title names the issue it closes. The step that moved Next
Step into Completed Steps and pulled the top of Future Steps into Next
Step goes with it. The entries already in Completed Steps stay.

Model: opus-5-5

TODO.md

@@ -2,10 +2,9 @@
 
 - `git pull` `next` and cut a branch from it — one branch per issue, named
   `issue-<N>-<slug>`. Never branch from `main`.
-- Do the work as one commit whose title ends with ` (closes #N)`, with the
-  `TODO.md` update in that same commit.
-- Move Next Step to the top of Completed Steps; move the top item of Future
-  Steps into Next Step.
+- Do the work as one commit whose title ends with ` (closes #N)`.
+- Add no entry to Completed Steps. What landed is recorded on the tracker and in
+  `git log`, where each commit title names the issue it closes.
 - Run `make fmt`, then `make check`. A feature branch may be red; `next` and
   `main` may not.
 - Rebase onto current `next` immediately before pushing — other branches land on