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/ahrefs.md

@@ -1,15 +1,32 @@
-# Ahrefs
+# Ahrefs MCP
 
-> TODO: Document tool usage for this skill
+Use for **top-pages-by-traffic** views on competitor sites, **content decay** candidates on the target, and Korean platform refdomains. Namespace: `mcp__ahrefs__*` / `mcp__claude_ai_Ahrefs__*`.
 
-## Available Commands
+## Key endpoints
 
-- [ ] List commands
+- `site-explorer-top-pages` — pages ranked by traffic for any domain (target or competitor)
+- `site-explorer-pages-history` — trend in indexed pages over time (decay signal)
+- `site-explorer-pages-by-traffic` — sorted by current traffic
+- `site-explorer-pages-by-internal-links` — informs internal-linking opportunities
+- `keywords-explorer-overview` / `-related-terms` / `-matching-terms` — topic expansion
+- `gsc-pages` — first-party top pages when GSC is connected to the project
 
-## Configuration
+## Example
 
-- [ ] Add configuration details
+```
+mcp__ahrefs__site-explorer-top-pages(target="<competitor>", country="us", limit=100)
+mcp__ahrefs__site-explorer-pages-history(target="<target>", history="monthly")
+mcp__ahrefs__keywords-explorer-related-terms(keyword="<topic>", country="us", limit=50)
+```
 
-## Examples
+## Strengths for this skill
 
-- [ ] Add usage examples
+- Best ranked-by-traffic view of competitor content inventory.
+- Historical pages-history surfaces decaying content quickly.
+- Pairs well with OurSEO crawl (which gives the on-page side).
+
+## Not for this skill when
+
+- **On-page content inventory of the target** — OurSEO `crawl_website` is the ground-truth source.
+- **Cannibalization detection** on the target — GSC first-party data is required.
+- **Korean Naver content** — Ahrefs underrepresents Naver Blog/Cafe; use `our research naver *` and WebSearch.