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/27-seo-ai-visibility/desktop/tools/semrush.md

@@ -0,0 +1,21 @@
+# Semrush MCP
+
+**Not applicable** for AI search visibility / Brand Radar today. Semrush does not expose an equivalent product as of 2026-05.
+
+Listed in this skill's `allowed-tools` only for forward compatibility if Semrush ships an AI-visibility product.
+
+## Do not use Semrush for
+
+- AI mentions / impressions / share of voice in AI answers
+- Cited domains / cited pages in AI responses
+- AI engine breakdown (ChatGPT / Gemini / Perplexity / Claude)
+- Brand SOV history in AI search
+
+## Use instead
+
+- **Ahrefs Brand Radar** (`mcp__ahrefs__brand-radar-*`) — the only viable backend for the core capability today.
+- **OurSEO `monitor_brand`** — for general web-mention monitoring (not AI-search specific) as a supplement.
+
+## If a Semrush AI product ships
+
+Update this stub and re-evaluate the Data Source Selection in `SKILL.md`. Until then, keep Brand Radar as the default and do not attempt to substitute Semrush.