Build an automated (or semi-automated) staleness check that leverages the summary layer: - For each doc summary, cross-check its claims against current reality — e.g. embed the summary and find the nearest CODE chunks; if a summary asserts a mechanism/name/flag that no longer exists (fails a symbol/lookup or contradicts the code), flag the doc as possibly stale. - Surface flags in a report (agent-tool docs-staleness) and/or the web Search/Docs UI (a "possibly stale" badge on a doc). - Optionally re-generate a doc summary and diff against the stored one after a code change to detect drift.

This makes the docs-freshness audit ONGOING instead of a one-time sweep.

Acceptance: - A tool/report lists docs whose summary claims are contradicted by current code (with the specific contradicted claim). - Low false-positive rate on a spot-check; flags map to genuinely stale content. - Runs off the existing kind=summary index (no new corpus).

Depends on: a complete summary index (kind=summary rebuild). Relates to: the docs-freshness audit ticket, and Vector Search milestone 27c526c72b.