feat(seo-skills): multi-backend Data Source Selection (#7)

Replaces single-vendor (Ahrefs-only) tool defaults with a per-task
backend menu across all 14 SEO skills. Each skill now lists every
capable MCP in allowed-tools and documents how to pick between
Semrush, Ahrefs, OurSEO Agent (CLI + MCP), DataForSEO, and GSC
in its SKILL.md Data Source Selection section.

Tool stubs (~40 new files) populated per skill with capability
deltas, call patterns, and explicit "not for this skill when"
callouts so the menu is self-correcting.

Skills affected: 19-keyword-strategy, 20-serp-analysis,
21-position-tracking, 22-link-building, 23-content-strategy,
24-ecommerce, 25-kpi-framework, 26-international, 27-ai-visibility,
28-knowledge-graph, 31-competitor-intel, 32-crawl-budget,
33-migration-planner, 34-reporting-dashboard.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

custom-skills/23-seo-content-strategy/desktop/tools/gsc.md

@@ -0,0 +1,42 @@
+# Google Search Console
+
+**Required** for two content-strategy tasks that no other backend can do honestly:
+
+1. **Cannibalization detection** — query × page combos where multiple URLs share impressions.
+2. **Decay detection** — period-over-period impression decline on the target's existing pages.
+
+## CLI commands
+
+```bash
+# Top queries — what the site already ranks for
+our research search-console queries --site sc-domain:<domain> --days 28
+
+# Top pages by clicks/impressions
+our research search-console pages --site sc-domain:<domain> --days 28
+
+# Combined query × page — REQUIRED for cannibalization
+our research search-console combined --site sc-domain:<domain> --days 90
+```
+
+`sc-domain:` prefix for Domain-verified properties; URL-prefix properties use the plain URL.
+
+## Decay workflow
+
+1. Pull `pages` for the last 90 days.
+2. Pull `pages` for the same 90-day window from 90 days prior (or use cached prior runs).
+3. Diff impression / click counts per URL.
+4. Flag URLs with >30% impression decline as decay candidates.
+5. Cross-reference with `our collect crawl` output to check freshness signals (last-modified, content age).
+
+## Cannibalization workflow
+
+1. Pull `combined` over 90 days.
+2. Group by query; count distinct URLs per query.
+3. Flag queries where 2+ URLs each have >100 impressions — the site is competing with itself.
+
+## Not for this skill when
+
+- **Net-new content discovery** — GSC only knows what already ranks. Use Semrush/Ahrefs for gap discovery.
+- **Competitor performance** — single-site only.
+
+Gotcha: `our-claude-skills/custom-skills/15-seo-search-console/code/gotcha/gsc-cli-integration.md`.