docs(backlog): clear the tail, keep what was in it

PLN: "deliberate killed tail was just a agent log or srth, capture that removed
as learning, nut then let me clean that backlog indeed was gboing tedious."

The tail is cleared and the new AI ENGINEER candidate section stands. But the
543 lines were not a log: they were two sessions of rig notes from 6-7
September, and six items in them were still open. Losing them to a backlog
cleanup is the quiet kind of loss — nobody misses a note they have forgotten
writing.

So the whole block is archived verbatim in docs/, with the open items hoisted
to the top of it where they can be read without scrolling. The one that
matters: `sudo tools/install-protect.sh` was never run, so the OOM-protection
daemon has never actually protected anything — it reported ok only because
scsynth and sclang already sat at the target value, and no write was attempted.