feat(seo-data): C2 — cannibalisation from Google's own data, one param away

The inventory called this "no duplicate-content / cannibalisation detection".
Splitting that into its two halves shows one is free and the other is a trap.

CANNIBALISATION — free, and the data was already reachable. Search Analytics
has always accepted several dimensions at once ("no limit to the number of
dimensions that you can group by"); this engine only ever sent
`"dimensions": [dim]` and _norm_queries only ever read `keys[0]`. So
query+page — the pairing that exposes the conflict — was one parameter away
and nobody asked. Same shape of win as W1.

  fetch.sh cannibal → queries where 2+ of OUR pages compete, ranked by total
  impressions, strongest page first inside each. Same auth, same quota family,
  no new scope. `capped` reports a full row window rather than presenting a
  truncated list as exhaustive — same rule as COVERAGE and the sitemap caps.

Grouping happens in the engine, deterministically: asking an LLM to group
1000 rows by query is arithmetic it should never be handed.

Backward compatible: rows gained `keys` (the list the API actually returns);
`key` stays as keys[0], so the single-dim quick-wins consumer is untouched.
A test pins both.

30/70 DUPLICATION — deliberately NOT built, and this is the honest half.
Measuring it needs main-content extraction (strip nav/header/footer). Without
that, comparing two same-template pages returns ~95% similar for every site —
a confident false positive, which is exactly the failure class the rest of
this branch exists to remove. It stays an explicit LLM judgement over the >=3
same-family pages C1c now samples for it, labelled as judgement, never quoting
a similarity percentage nobody computed. A wrong number would be worse than
the current honest gap.

The two must not be merged in the report either: cannibalisation is a SERP
fact Google measured; 30/70 is a content question. The spec now says so.

Verified: fixture with 3 pages on one query, 2 on another, 1 on a third →
2 conflicts, correct ranking, single-page query excluded; live dispatch
degrades cleanly with no account; seo-data 110 -> 119 pass, 0 fail; full suite
green; shellcheck + py_compile clean.

agents/seo-analyzer.md

@@ -340,8 +340,37 @@ When STEP 0/STEP 1 recorded a GSC account+property (not "none"):
 ```bash
 bash ~/.claude/lib/seo-data/fetch.sh queries --account "$GSC_ACCOUNT" --property "$GSC_PROPERTY" --days 90 --dim query
 bash ~/.claude/lib/seo-data/fetch.sh inspect --account "$GSC_ACCOUNT" --property "$GSC_PROPERTY" --url "https://$DOMAIN/"
+bash ~/.claude/lib/seo-data/fetch.sh cannibal --account "$GSC_ACCOUNT" --property "$GSC_PROPERTY" --days 90
 ```
 
+**`cannibal` — keyword cannibalisation, from Google's own data (C2).** Groups
+90 days of `query`+`page` rows and returns every query where 2+ of OUR pages
+compete, ranked by total impressions. The API always allowed multiple
+dimensions; this system only ever asked for one, so the conflict was invisible.
+
+Read it:
+- `conflicts[]` → for each, the strongest page (most impressions) is listed
+  first. That is usually the one to KEEP; the others either consolidate into
+  it (301 + merge content) or get differentiated. Never "fix" this by deleting
+  a page that has clicks — say what competes and let the user choose.
+- A conflict with a large impression total and every page beyond position 10
+  is the real prize: Google can't decide which page to rank, so none rank.
+- `capped: true` → the row window was full; there are conflicts past the cut.
+  Say so in §14 rather than presenting the list as exhaustive.
+- `status: degraded` → no GSC account. Cannibalisation is then **not
+  auditable** — no substitute exists on-site. §14 line, do not guess it from
+  title similarity.
+
+**This is NOT the 30/70 rule, and do not merge the two.** Cannibalisation is
+a SERP fact Google measured. The 30/70 duplication rule is a content-similarity
+question with **no data source here**: measuring it properly needs main-content
+extraction (strip nav/header/footer), and without that a naive comparison of
+two same-template pages returns ~95% similar for every site, which is a
+confident false positive. So 30/70 stays an explicit LLM judgement over the
+≥3 same-family pages STEP 5 now samples for it — label it as judgement in the
+report, never as a measurement, and never quote a similarity percentage you
+did not compute.
+
 Report: top queries; flag **QUICK WINS** = rows with position between 4
 and 10 AND high impressions (candidates to push onto page 1 with a
 title/meta/content tweak). Report index coverage from `inspect`. All