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

@@ -671,9 +671,6 @@ but letters and digits, or when it is longer than 16 characters.
 When the date or the time zone changes, the next run stores the original at its
 new path and leaves the old copy where it is.
 
-A file deleted in Ente keeps its original at its save path, and the JSON beside
-it. The next run removes its symlinks from the collection directories.
-
 The library gives the same path as `photo.savePath` and writes it with
 `photo.download()` (see "A photo's content").