fix(seo): I2 — remove VSI, an SEO-blog fiction, from CWV thresholds

seo-analyzer.md:278 listed "VSI (Visual Stability Index) — new 2026 signal,
Google Core Web Vitals 2.0" as a threshold, stated as fact, no hedge, in
client-facing audits. It does not exist.

Verified against two primary sources:
- developer.chrome.com/docs/crux/api — complete metric list carries no
  visual_stability_index. Blogs claimed "Google is actively collecting VSI
  through CrUX": flatly false.
- web.dev/articles/vitals — three stable CWV (LCP, INP, CLS). No VSI, no
  "Core Web Vitals 2.0". Thresholds change with prior notice on an annual
  cadence.

Ten SEO blogs cross-cited each other into an apparent consensus. WebSearch
returns that consensus, which is why the resources README rule "agents MUST
cross-check via WebSearch on FULL" did not catch it — that mitigation
launders blog misinformation into apparent verification.

Fix removes the metric and states the sourcing rule where a future rumour
would land: primary sources only (web.dev / Chromium blog / CrUX API list,
the last being decisive — a metric CrUX cannot return is one we cannot
score). Incident documented inline so it is not re-added.

Verified: make test 35 GREEN / 0 RED.

agents/seo-analyzer.md

@@ -275,8 +275,21 @@ Evaluate each present/missing:
 - **LCP** (Largest Contentful Paint) — < 2.5s
 - **INP** (Interaction to Next Paint) — < 200ms (replaced FID in Mar 2024)
 - **CLS** (Cumulative Layout Shift) — < 0.1
-- **VSI** (Visual Stability Index) — new 2026 signal, Google Core Web
+
-  Vitals 2.0
+**Core Web Vitals are exactly these three** (web.dev/articles/vitals,
+verified 2026-07-16). Google ships threshold changes with prior notice on a
+predictable annual cadence — a "new CWV" that only SEO blogs know about does
+not exist. Before adding a metric here, confirm it against a PRIMARY source:
+web.dev, the Chromium blog, or `developer.chrome.com/docs/crux/api` — that
+API metric list is decisive, because a metric CrUX cannot return is a metric
+we cannot score.
+
+**WebSearch is not confirmation.** SEO blogs cross-cite each other into fake
+consensus. A "VSI (Visual Stability Index) — new 2026 signal, Core Web
+Vitals 2.0" line lived here until 2026-07-16 on exactly that basis: ten
+blogs asserted it, several claimed CrUX was already collecting it, and it is
+absent from both the CrUX API metric list and web.dev. Stated as fact, in a
+threshold list, in client-facing audits.
 
 When a GSC account+property were passed in context, fetch CrUX field
 data first (**tilde path mandatory** — this agent runs from the