TODO.md: open work only (closes #235)

Removes the Completed Steps section: dated accounts of finished work,
review and merge history, and notes on past decisions. Git keeps the
history. Status, Next Step and Future Steps are unchanged.

The Workflow step "move Next Step to the top of Completed Steps" now
reads "delete Next Step from `TODO.md`", the standard wording. The
`README.md` TODO section no longer calls `TODO.md` the work log; it
says the file holds the next step and the steps planned.

Model: opus-5-5

README.md

@@ -1935,8 +1935,7 @@ These apply to people and to coding agents alike.
 
 ## TODO
 
-`TODO.md` in the repo root is the work log: the next step, the steps done and
-the steps planned. Open now:
+`TODO.md` in the repo root holds the next step and the steps planned. Open now:
 
 - [ ] Check quak against a live Ente account. This needs an account only sneak
       has.