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

Remove the Completed Steps log, the Status section and the finished
Future Steps items. The Workflow now says to delete Next Step once done
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 from `TODO.md`
+- delete Next Step once its work is done: this file lists open work only
 - `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
@@ -14,10 +14,6 @@
   `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
@@ -27,5 +23,6 @@ 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