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/22-seo-link-building/desktop/tools/semrush.md

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+# Semrush MCP
+
+Alternative backlink backend. Use when the user is already in Semrush context, wants a second opinion, or doesn't have an Ahrefs subscription. Ahrefs remains the default for backlink work in this skill.
+
+Call pattern: discovery → `get_report_schema` → `execute_report`.
+
+## Key reports
+
+- `backlink_research` — referring domains, backlink count, anchor distribution
+- Authority Score (Semrush's equivalent of Ahrefs DR)
+- Toxic Score for risk flagging
+
+## Example
+
+```
+mcp__semrush__backlink_research(query="<domain>", database="us")
+mcp__semrush__get_report_schema(report_id="<id>")
+mcp__semrush__execute_report(report_id="<id>", params={...})
+```
+
+## When to use over Ahrefs
+
+- User explicitly prefers Semrush data.
+- Cross-vendor verification ("does Semrush agree with Ahrefs on this referring domain?").
+- Authority Score weighting instead of DR.
+
+## Not for this skill when
+
+- **Default backlink work** — Ahrefs has the stronger backlink graph; use it unless the user named Semrush.
+- **Korean platform refdomains** — both Semrush and Ahrefs underrepresent Naver Blog/Cafe/Tistory; supplement with WebSearch.