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/25-seo-kpi-framework/desktop/tools/ahrefs.md

@@ -1,15 +1,37 @@
-# Ahrefs
+# Ahrefs MCP
 
-> TODO: Document tool usage for this skill
+In the KPI framework, Ahrefs owns **backlinks/DR**, contributes alternates for **traffic value** and **visibility**, and provides historical trend series. Namespace: `mcp__ahrefs__*` / `mcp__claude_ai_Ahrefs__*`.
 
-## Available Commands
+## KPI dimensions served by Ahrefs
 
-- [ ] List commands
+| KPI | Endpoint |
+|---|---|
+| Domain Rating | `site-explorer-domain-rating` |
+| DR history (trend) | `site-explorer-domain-rating-history` |
+| Backlink count + dofollow ratio | `site-explorer-backlinks-stats` |
+| Referring domains (count + trend) | `site-explorer-refdomains-history` |
+| Organic traffic estimate | `site-explorer-metrics` |
+| Metrics history (trend) | `site-explorer-metrics-history` |
+| Top pages traffic distribution | `site-explorer-top-pages` |
+| First-party clicks/impressions (if GSC connected) | `gsc-performance-history`, `gsc-positions-history` |
 
-## Configuration
+## Example
 
-- [ ] Add configuration details
+```
+mcp__ahrefs__site-explorer-metrics(target="<domain>")
+mcp__ahrefs__site-explorer-metrics-history(target="<domain>", history="weekly")
+mcp__ahrefs__site-explorer-domain-rating-history(target="<domain>", history="weekly")
+```
 
-## Examples
+## Strengths for this skill
 
-- [ ] Add usage examples
+- Backlink dimension is unmatched.
+- Trend series are pre-computed — useful for period-over-period KPI charts.
+- When GSC is connected to the Ahrefs project, first-party CTR / position history is available.
+
+## Not for this skill when
+
+- **Technical-health KPI** — OurSEO's audit + crawl is deeper.
+- **GA4 / on-site engagement KPI** — `our research ga4 *` only.
+- **GBP local KPI** — `our collect gbp *` / `our audit local` only.
+- **Indexed pages count** — OurSEO `check_index` / GSC Coverage.