TODO.md: remove the work log, keep only open work (closes #167)

Drop the Completed Steps log and the finished "commit this TODO.md"
item. Git keeps the history. The Workflow section now says to delete
the finished Next Step, as the standard Workflow block does.

Model: opus-5-5

TODO.md

@@ -2,7 +2,7 @@
 
 - branch (from `main`)
 - do the work in Next Step
-- move Next Step to the top of Completed Steps
+- delete Next Step from `TODO.md`
 - move the top item of Future Steps into Next Step
 - commit (`TODO.md` changes in the same commit as the work)
 - merge to `main` if the branch is not protected, otherwise open a PR