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

TODO.md

@@ -27,11 +27,5 @@ P2: security: per-origin rate limiting
 - P2: HTTP response handling
     - Last-Modified headers
 - P2: operational
-    - Prometheus metrics still missing:
-        - cache size on disk and its limit
-        - evictions
-        - upstream fetch duration and errors
-        - transcode duration
-        - images in processing
     - measure the 1k to 5k req/s target with `script/loadtest` on a machine not
       shared with other work