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

@@ -27,5 +27,11 @@ 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