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/ourseo.md

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+# OurSEO Agent (CLI + MCP)
+
+OurSEO is **not an AI-visibility backend.** Use it here only as a supplement for general brand-monitoring work that complements Ahrefs Brand Radar.
+
+## What OurSEO covers
+
+```bash
+# Web-wide brand mention scan via Google Custom Search
+our research google brand "<brand>" --exclude <own.com>
+
+# Korean engines (Naver brand presence)
+our research naver serp "<brand>"
+```
+
+MCP equivalent:
+
+```
+mcp__ourseo__monitor_brand(brand="<brand>", exclude_domains=["<own.com>"])
+```
+
+## How OurSEO + Brand Radar pair
+
+- **Brand Radar** (Ahrefs) → AI search citations, impressions, SOV, cited pages
+- **OurSEO monitor_brand** → standard Google search brand mentions across the web
+
+Together they show:
+- "Where are we cited in AI vs the open web?"
+- "Are the high-citation pages in AI also the ones earning standard organic links?"
+- "Does our share of voice in AI match our share in Google brand SERPs?"
+
+## Not for this skill when
+
+- **AI mentions / impressions / SOV / cited domains/pages** — Brand Radar only.
+- **AI engine breakdown** — Brand Radar `brand-radar-ai-responses`.
+
+## Important
+
+Treat OurSEO output as a **second axis**, not a substitute. Reporting AI visibility using only `monitor_brand` would mislead the user — those are open-web mentions, not AI citations.

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

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+# 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.