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/semrush.md

@@ -0,0 +1,34 @@
+# Semrush MCP
+
+Use for **e-commerce keyword discovery**, **organic competitor research**, and **third-party site audit** of stores.
+
+Call pattern: discovery → `get_report_schema` → `execute_report`.
+
+## Key reports
+
+- `keyword_research` — product / category term volume + KD + intent
+- `organic_research` — competitor store organic keywords + top pages
+- `overview_research` — store-level traffic / visibility
+- `siteaudit_research` — store-wide technical SEO audit
+
+## Example
+
+```
+mcp__semrush__keyword_research(query="<product term>", database="us")
+mcp__semrush__organic_research(query="<competitor store>", database="us")
+mcp__semrush__siteaudit_research(query="<your store>", database="us")
+```
+
+Korean market: `database="kr"`.
+
+## Strengths for this skill
+
+- Cleanest competitor-store organic view (which product terms drive their traffic).
+- Site audit catches site-wide technical issues quickly.
+- `keyword_research` is the default for product/category term expansion.
+
+## Not for this skill when
+
+- **Product schema validation** — OurSEO owns this.
+- **Korean marketplace presence** — WebSearch only.
+- **Catalog crawl with custom configuration** — OurSEO `crawl_website`.