docs: measurements in public history carry numbers, not provenance

A release audit found hostnames, store identities and operational anecdotes in
this branch's commit messages — not trade secrets, but nothing a public
repository's permanent history should carry either. The messages were rewritten
to keep every number and drop every provenance; the rule is written down here
so the next measurement does not have to be caught by an audit.

CONTRIBUTING.md

@@ -57,6 +57,13 @@ see `package.json` for `test:integration`, `test:coverage`, and friends.
   description states a number, cite the benchmark that produced it (see
   [docs/performance-envelopes.md](docs/performance-envelopes.md) for the
   pattern). Don't state an estimate as if it were measured.
+- **Measurements carry numbers, not provenance.** Public commit messages and
+  docs give the SHAPE a number was taken at and never where it was taken: no
+  hostnames, no store or deployment identities, no operational anecdotes about
+  someone's running system. "A 14,056-noun / 72,679-verb production-shaped
+  store, measured solo under an exclusive lock" tells a reader everything the
+  number depends on; the machine it ran on and whose data it was tell them
+  nothing except where somebody's infrastructure lives.
 
 ## License