docs: the fallback grep, and there were three consumers not two

Records the second sweep alongside the comment grep, because they find different
populations. The comment grep catches defects someone documented with an
apologetic note; the fallback grep catches the ones nobody commented, which are
more common. It needs triage -- `unwrap_or(true)` as "no filter given" is
correct and everywhere -- and the signal is a fallback in a reader path.

Also sharpens the "adding a reader is not the same as adopting it" note. There
were three consumers of the macOS memory path, not two: the ontology adopted the
real reader in 5.2.0, `MonitoringBackend` kept fabricating, and `ai_api::tools`
kept a third hand-rolled copy with a hardcoded 16384-byte page size. Count the
consumers before assuming a migration finished.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>