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 Prometheus item names the metrics still missing. 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

@@ -28,9 +28,8 @@ P2: security: per-origin rate limiting
     - Last-Modified headers
 - P2: operational
     - Prometheus metrics still missing:
-        - cache hits and misses, split into source images and variants
         - cache size on disk and its limit
-        - evictions and the bytes they free
+        - evictions
         - upstream fetch duration and errors
         - transcode duration
         - images in processing