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/20-seo-serp-analysis/desktop/tools/ahrefs.md

@@ -1,15 +1,24 @@
-# Ahrefs
+# Ahrefs MCP
 
-> TODO: Document tool usage for this skill
+Use when the user explicitly wants Ahrefs' SERP view, or already has an Ahrefs project for the domain. Namespace: `mcp__ahrefs__*` (Desktop) / `mcp__claude_ai_Ahrefs__*` (Claude.ai) — same backend.
 
-## Available Commands
+## Key endpoints
 
-- [ ] List commands
+- `serp-overview` — top organic, SERP features, paid layout for a keyword
+- `rank-tracker-serp-overview` — SERP layout for keywords in a tracked project
+- `keywords-explorer-overview` — pairs with SERP for SERP-feature signals (Featured Snippet, PAA)
 
-## Configuration
+## Example
 
-- [ ] Add configuration details
+```
+mcp__ahrefs__serp-overview(keyword="<keyword>", country="us")
+mcp__ahrefs__serp-overview(keyword="<keyword>", country="kr")
+```
 
-## Examples
+## Not for this skill when
 
-- [ ] Add usage examples
+- **Naver SERP** — Ahrefs doesn't cover Naver. Use `our research naver serp`.
+- **Single-keyword position spot-check** — `mcp__ourseo__check_serp` is cheaper.
+- **Full SERP JSON with all features for export** — DataForSEO via `our serp live`.
+
+Reference: call `mcp__ahrefs__doc` once per session before first use.