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

@@ -2,7 +2,7 @@
 
 - branch per issue from `next`
 - do the work in Next Step
-- delete Next Step once its work is done: this file lists open work only
+- delete Next Step from `TODO.md`
 - `TODO.md` merges with git's union merge (`.gitattributes`), which never
   reports a conflict: read the merged entries after every merge or rebase
 - move the top item of Future Steps into Next Step
@@ -14,6 +14,10 @@
   `next` into `main`, via the single milestone PR
 - push
 
+# Status
+
+pre-1.0. The `1.0.0` milestone is in progress.
+
 # Next Step
 
 P2: security: per-origin rate limiting
@@ -23,6 +27,5 @@ P2: security: per-origin rate limiting
 - P2: HTTP response handling
     - Last-Modified headers
 - P2: operational
-    - Prometheus performance metrics
     - measure the 1k to 5k req/s target with `script/loadtest` on a machine not
       shared with other work