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/24-seo-ecommerce/desktop/tools/ahrefs.md

@@ -1,15 +1,30 @@
-# Ahrefs
+# Ahrefs MCP
 
-> TODO: Document tool usage for this skill
+Use for **product/category backlinks** and third-party **site audit** views on e-commerce sites. Namespace: `mcp__ahrefs__*` / `mcp__claude_ai_Ahrefs__*`.
 
-## Available Commands
+## Key endpoints
 
-- [ ] List commands
+- `site-explorer-top-pages` — which product/category pages earn the most links/traffic
+- `site-explorer-pages-by-backlinks` — link equity distribution across the catalog
+- `site-audit-issues` — schema, canonical, parameter-URL issues from Ahrefs' crawler
+- `site-audit-page-explorer` — per-page issue surfacing
+- `keywords-explorer-overview` — product-term volume / KD
 
-## Configuration
+## Example
 
-- [ ] Add configuration details
+```
+mcp__ahrefs__site-explorer-top-pages(target="<store>", country="us", limit=100)
+mcp__ahrefs__site-audit-issues(project_id="<id>")
+mcp__ahrefs__keywords-explorer-overview(keyword="<product>", country="us")
+```
 
-## Examples
+## Strengths for this skill
 
-- [ ] Add usage examples
+- Surfaces which **product pages** earn external links — informs which products deserve hero-page promotion.
+- Site Audit catches canonical / parameter-URL issues across the catalog without running your own crawler.
+
+## Not for this skill when
+
+- **Product schema validation** — OurSEO `audit_page` + `build kg-schema` owns this end-to-end.
+- **Korean marketplace presence** (Naver Smart Store, Coupang, Gmarket, 11번가) — Ahrefs doesn't cover these. Use WebSearch.
+- **Live catalog crawl of the target** — OurSEO `crawl_website` is more configurable.