Keep only open work in TODO.md (closes #348)

Remove the Completed Steps log, the history in the Status section and
the finished Future Steps items. Status keeps the current state only.
The Workflow says to delete Next Step from TODO.md instead of moving it
to Completed Steps. README no longer points at the load-test
measurement that lived in the log.

Model: opus-5-5

README.md

@@ -1090,9 +1090,7 @@ For each scenario the script prints vegeta's report and two lines of its own:
   request for `miss`, and one per image for `herd`, that is the requests sent
   divided by the number of clients.
 
-The numbers depend on the machine and on whatever else runs on it. The first
-measurement, made on a shared machine with few clients, is in `TODO.md`; it says
-nothing about the target.
+The numbers depend on the machine and on whatever else runs on it.
 
 `script/bench` (or `make bench`) measures cache hits alone, without the network
 and with no pixad process: a Go benchmark in `internal/handlers` serves one